Ships requistioned by the Royal Navy from British citizens during WWI fell into two main categories: liners requisitioned for use as Armed Merchant Cruisers (AMCs) and trawlers, yachts and drifters hired for patrol duties. The former category included several ships that had been built using money loaned or invested by the British government. This came with the stipulation that, in event of war, these ships would be turned over to the Royal Navy for conversion into AMCs. Such ships were typically constructed with reinforced decks and fittings for guns, to ease conversion. Ships that fit into this category include the Cunard liners Mauritania and Lusitania, though they were never really used as AMCs. As part of an agreement signed in 1903, Cunard were paid £150,000 per year, in return for putting these two ships at the disposition of the RN, with the International Mercantile Marine Co receiving similar payments for the potential use of White Star Line ships. Otherwise, if a shipping line wished to take a contract with the Royal Mail to carry mail, its ships would have to be made available to the RN in time of war, Subsidies were also granted to ships that might have been useful Either way, the owners were well compensated prior to the war beginning. Whether or not the ship was built with RN funds, the ships were hired from their owners. An example would be the P&O liner Macedonia, which cost the RN £1,698 per week. Rates for the hire of Cunard and IMM ships were set by the 1903 agreement, with the Cunard ships being available at a rate of 25 shillings per month per ton. The ships were also available for outright purchase, at a rate of 110% of their value at time of purchase. In addition, the owners would be compensated when a hired ship was lost. For example, the White Star Line's Oceanic was estimated, in 1905, to cost £705,617 if she were to be lost. Meanwhile, owners of smaller ships, whether hired or requisitioned, were also compensated. Hire rates were initially set at 12 per cent per registered ton, with four percent depreciation from the original cost per year. However, this was found to be insufficient, and it was swiftly raised to 18 per cent. Typical pay for trawler hire by the RN was £100 per month.

Several ships under construction in Britain were also requistioned or purchased from foreign governments. The most important were the Ottoman battleships, Sultan Osman I and Reşadiye, the Chilean battleships Almirante Cochrane and Almirante Latorre, and the Italian liner Conte Rosso. Several destroyers, cruisers and monitors were also requisitioned, from navies including the Greek, Portuguese and Brazilian. In the case of the Ottoman ships, no compensation was paid, as the Ottoman declaration of war came soon after their requisitioning. Almirante Cochrane and Conte Rosso were purchased while incomplete, and both were converted into carriers, with the former becoming HMS Eagle and the second becoming HMS Argus. Almirante Latorre was purchased by the RN in 1914, and served as HMS Canada. She was sold back to Chile in April 1920, at a cost just a third of her original value.