Following the outbreak of the First World War in August 1914, the squadron conducted operations in the English Channel, and was based at Sheerness to guard against a possible German invasion.

In addition, the new design incorporated longer (and thus more powerful) main and secondary guns and an improved hull form.

The Formidable-class ships were powered by a pair of 3-cylinder triple-expansion engines, with steam provided by twenty Belleville boilers.

Captain Alexander William Chisholm-Batten paid off HMS Resolution on 9 October and he and his crew turned over to Formidable on the following day.

[3] In September 1902 she visited the Aegean Sea with other ships of the station for combined manoeuvres near Nauplia,[4] and in January the following year she cruised in the water off Syracuse, Sicily.

[7][8] Paid off at Chatham Dockyard on 17 August 1908, Formidable began another refit and recommissioned on 20 April 1909 for service in the 1st Division, Home Fleet, stationed at the Nore.

Bayly suspected no danger from submarines, and so steamed his ships in line ahead formation at a speed of 10 knots (19 km/h; 12 mph).

Unknown to the British, the German submarine U-24 stalked the squadron while it was exercising all afternoon, trying to find a suitable attack position.

By the time Topaze closed with Formidable twenty minutes later, the latter vessel had already taken on a list of 20 degrees to starboard, and Loxley had issued the order to abandon ship.

Men attempting to save the vessel remained aboard and through counter-flooding reduced the list, though Formidable was by then very low in the water.

Topaze, joined by Diamond, began the rescue effort, but the heavy seas made it very difficult to bring men aboard.

[12][15] An inquiry from the Admiralty into the sinking determined that the risk of conducting training exercises in the Channel without destroyer protection was excessive and should not be continued.