This is very much a hypothetical question, and therefore doesn’t really have a clear answer. There are a couple of significant qualifying elements. Firstly, when? The war is six years long and much changes. Secondly, how can you divorce the conjecture of relative strength from the strategic situations of the period? For example, how relevant is it to compare the Royal Navy of late 1939 with Japanese Navy of the same time? One is at war, the other will not be for 2 years. Likewise how relevant is it to compare the Japanese navy of late 41 with the Royal Navy of 41. The latter having been at war with two other powers for 2 years(ish)? Is it relevant to consider a war between Japan and Britain without considering the position of the naval powers? Are we talking in isolation or as part of the wider sequence of the war?

As of 1939 the Royal Navy was the largest and arguably the most powerful navy by quite some margin over any other naval power, including the US Navy. Much can be read and written regarding the merits and constraints of the interwar naval treaty system, yet I believe it is general historical consensus that Britain only agreed to parity with the US in name only. Throughout the 1920s the Royal Navy was significantly larger the US Navy and larger than Italy and Japan combined. The events of the early 1930s, especially the London Naval Treaty of 1930 altered this to varying degrees dependant on who you read. Simplistically Britain agreed to extend parity with the US to a greater extent than before, new German designs prompted the French to withdraw from system and start to build new ships prompting Italy to do the same. At the same time Japan began to systematically subvert the limitation system while still nominally a part of it. This provides a period of time between 1930 and some point in the mid 30s (some suggest as early as the Invasion of Manchuria and the establishment of the Defence Requirement Committee in 1933, others closer to 35’) where France, Italy and Japan are modernising and rearming and Britain was not. After the collapse of the second London Treaty and the program of rearmament Britain builds at a rate to quickly restore those similar ratios by the 1939.

The British Parliamentary Command Papers (Cmnd) ‘Fleets’, produced every year for Parliament through a mixture of intelligence, open source information and official returns to the League of Nations and the signatories to the naval limitations treaties illustrate (a slightly incomplete – Some Japanese building is omitted through duplicity, specifically including Yamato/Musashi, which I have added back in) picture of strength up to Jan 1939 for Britain, France, Italy, Germany, Japan and the Soviet Union. It shows the continued numerical dominance of Britain over the US or Japan and Italy combined. What it does show is that total strength is less than Japan/Italy/Germany, and that French strength combined with Britain’s is greater than the other 3. It also shows that of the most modern ships (those built in the last 10 years) the ratio is narrower, especially in Battleships. (Laid down by Jan 19: Britain 7, France 6, Germany 4, Italy 4, Japan 2, US 6). In contrast the most modern Cruisers and Carriers illustrate the large building programs of Britain, on paper at least retaining the ratios over Japan and Italy.

However all this is just numbers, there have been books and books written on relative qualitative strength of ships, some of it excellent, some of it coffee-table war porn. There are so many relative differences in class of ships, function, design and cost that I personally don’t focus too much on what unit was better than others as it tends to be appleVsoranges and mostly circumstantial and retrospective. BUT I think it is fair to say that ships designed and built in Britain in the 30s had a far greater collection of constraints put upon them than those of other navies. Britain was generally sticking to the rules of treaties closer than the others, generally with an eye to future peacetime use/cost/rules rather than definite war, and due to the slightly delayed nature of their rearmament, compromised on things that ideally they wouldn’t have having been dealt a hypothetical hand free from financial, political, diplomatic and strategic constraints.

Its worth noting that in wartime building no country other than the US built more than Britain. If you ignore losses (and this again is very hypothetical) Britain’s numerical strength Vs Italy/Germany/Japan increases during the years. It is also important not to overestimate US naval strength until much later in the war. It is not until the vast programs of fleet, light and escort carriers, and a huge cruiser program begin to appear and be felt does the balance of naval strength decisively and dramatically swing. Im no expert on US navy, but I would guestimate this not until late 43, maybe even 44.

Its also worth going back and talking about British interwar planning and expectations of war, especially with Japan and how that effected their planning and requirements. The ‘One power Standard’ has been written about extensively and generally is viewed by naval enthusiasts and naval older historians as some kind of evidence of British naval decline from a two power standard or even a three power standard. More recent work has illustrated just how flexible a concept of the ‘One power standard’ actually was. It changed and shifted over time to suit the political, financial, diplomatic and service realities. Simplistically in the early 20s it meant maintaining a healthy balance of power over the United States in aspects other than Battleships. As this became more unrealistic for a variety of reasons this moved to maintaining a standard against Japan. The standard is often misinterpreted to mean some kind of parity. When in practice it meant the ability to engage in an offensive war against One power while adequately maintaining peace against the rest. So the standard as applied to japan in the later 20s and early 30s was to conduct an offensive war against Japan while maintaining enough strength to match a second European power (assumed to be France or Italy). One of the aspects of the London Treaty that is much criticised by some naval historians is that it limited cruiser strength, one of the central pillars by which the Royal navy was to maintain superiority, and central to the trade based concepts of war with Japan. These war plans focus on the viability of Singapore as citadel to resist any initial Japanese attacks and then host a large build-up of naval forces before a mixture of trade warfare and fleet actions would bring about the conditions under which Japan could be loosely blockaded until they sued for peace. Far eastern war plans, Singapore, cruiser strengths etc and its impact on appeasement are still things that are debated today. They were much criticised at the time by people like Admiral Richmond, got absolutely pummelled in the initial historiography written in the 50s-70s, and now is being gradually re-examined and rehabilitated. Either way these various plans are predicated on 1) Singapore, 2) war with Japan alone, or possibly with another (France/Italy). I will leave Singapore out of this as that is a book in its own right. But simply the development of Singapore was never able to match requirements and there are some shaky strategic thinking regarding the whole concept. What is important to consider is that this was for an Offensive war against Japan. Once the situation of the 30s change the ratios and parameters this slowly shifts to a defensive one. Once German rearmament starts to focus British concerns Germany starts to become the ‘Another’ in a war with Japan. All well and good for Britain. But once the Abyssinian crisis of 35/36 and the Spanish Civil War put Italy firmly into the ‘Another’ camp as well then things become very difficult for Britain. No interwar planning (to my knowledge) had been based on the assumption of Italy/Japan/Germany even with French support.

p2

So, finally back to your question. Britain had been fully expectant to fight Japan aggressively in the 20s and early 30s. The realignment of European adversaries change this to a defensive war. In 1940 Britain still had enough forces, with the French, to fight Japan, Italy and Germany (at least defensively until war production shifted the balance in their favour). What was unexpected, or at least unfortunate was the attrition of submarines and Airpower to their existent strength. Again, while airpower is a book in itself, I don’t think anyone had quite grasped the extent of attrition aspects of airpower on navies. The Mediterranean was a grinder, not so much in ships lost, but in the constant sapping damage suffered by all ships, especially the major units. Early losses to U boats quickly altered slim ratios of forces. The loss of 2 fleet carriers before the fall of France has an impact on future operations. And do not underestimate the fall of France. Its also interesting to note that all of the various permutations of power standards and naval limitations do not attempt to control air power (perhaps obviously) so the balance of naval power as understood throughout the interwar period is artificial as it fails to account for airpower. They also barely account for submarines.

Long story short. In the context of late 1941 Britain was in no position to successfully fight Japan, Germany and Italy on its own. It had never intended to, never planned for it, and with the loss of France and the realities of this war it was unlikely ever to be able to. In this context, without the United States it is hard to imagine a scenario where Britain could fight an effective war against Japan after the loss of Singapore. If Singapore could have been held than maybe, eventually, years later. But that is entirely hypothetical, which I suppose is the point of your question. Hypothetically then I believe Britain would have been able to defeat Japan in isolation (defeat as in gain a commanding control of the sea eventually, not necessarily force a surrender). Airpower and logistics would have made it a much harder prospect than they planned for in the late 20s and it is harder to imagine how this would have looked like. With France, the prospects of facing Japan and Italy, long term is also good.

This is all guess work though. The reality is you cannot separate out these permutations. Britain had been prepared and probably capable (in the minds of the pre war estimation) of fighting Japan, but not in the context of the war as it was in late 1941.

There is a huge literature of the interwar navies, some useful ones focussing on the standards, policies and positions of Britain and the Royal Navy are:

Bell, Christopher M. At the Crossroads between Peace and War : The London Naval Conference in 1930, edited by John H. Maurer and Christopher Bell. Annapolis, United States: Naval Institute Press, 2014.

———. "‘How Are We Going to Make War?’: Admiral Sir Herbert Richmond and British Far Eastern War Plans." Journal of Strategic Studies 20, no. 3 (1997/09/01 1997): 123-41.

———. Royal Navy, Seapower and Strategy between the Wars. Basingstoke, UNITED KINGDOM: Palgrave Macmillan Limited, 2000.

Ferris, John. The Evolution of British Strategic Policy, 1919-26. London: Macmillan Press, 1989.

Roskill, Stephen. Naval Policy between Wars. Volume I : The Period of Anglo-American Antagonism 1919-1929. Barnsley: Seaforth Publishing, 2016.

———. Naval Policy between Wars. Volume Ii : The Period of Reluctant Rearmament 1930-1939. Barnsley: Seaforth Publishing, 2016.