When the First World War broke out, "Britannia" was transferred back to the 3rd Battle Squadron, which was assigned to the Grand Fleet, the main British fleet during the war. Through 1914 and 1915, the ships frequently went to sea to search for German vessels, but "Britannia" saw no action during this period. By the end of the year, the Grand Fleet stopped operating with the older 3rd Battle Squadron ships, and in 1916, she was attached to the 2nd Detached Squadron, then serving in the Adriatic Sea. After a refit in 1917, she conducted patrol and convoy escort duties in the Atlantic. On 9 November 1918, just two days before the end of the war, she was torpedoed by a German submarine off Cape Trafalgar and sank with the loss of 50 men. "Britannia" was one of the last British warships to be sunk in the war.