The keel of the RMS Gigantic was laid on November 30th, 1911 at the Harland and Wolff shipyard in Belfast, on the gantry slip previously occupied by the Olympic, 13 months after the launch of that ship. The acquiring of the ship was planned to be at the beginning of 1914. Due to improvements introduced as a consequence of the Titanic disaster, Britannic was not launched until February 26, 1914; which was filmed along with the fitting of a funnel. Several speeches were given in front of the press, and a dinner was organised in honour of the launching. Fitting out began subsequently. The ship entered dry dock in September and her propellers were installed.

Reusing Olympic's space saved the shipyard time and money by not clearing out a third slip similar in size to those used for Olympic and Titanic. In August 1914, before Britannic could commence transatlantic service between New York and Southampton, the First World War began. Immediately, all shipyards with Admiralty contracts were given top priority to use available raw materials. All civil contracts including the Britannic were slowed down. The naval authorities requisitioned a large number of ships as armed merchant cruisers or for troop transport. The Admiralty paid the companies for the use of their ships but the risk of losing a ship in naval operations was high. The big ocean liners were not taken for naval use, because smaller ships were easier to operate. RMS Olympic returned to Belfast on 3 November 1914, while work on her sister continued slowly.

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