They were armed with a battery of four 12-inch (305 mm) guns, they had top speed of 18 knots (33 km/h; 21 mph), and they marked the adoption of Krupp armour in British battleship designs.

By 1912, all three ships had been assigned to the 5th Battle Squadron, Home Fleet, where they remained until the outbreak of the First World War in August 1914.

On the night of 31 December 1914 – 1 January 1915 while on patrol in the Channel, the 5th Squadron encountered a German U-boat that torpedoed and sank Formidable.

Irresistible was sent to the Dardanelles Campaign in February 1915, and after engaging in a series of attacks on the Ottoman coastal fortifications, she struck a naval mine and sank.

Implacable, the sole surviving member of the class, joined the Dardanelles operations in March 1915 and saw action during the landings at Cape Helles in April.

The Board concluded that repeating the Canopus class would not be a workable solution, since the Imperial Japanese Navy had begun building larger, 15,000-tonne (15,000-long-ton; 17,000-short-ton) battleships and the Canopus design was too small to carry the new 40-calibre 12-inch (305 mm) Mk IX guns, which were significantly heavier than the shorter 35-calibre BL 12-inch Mk VIII guns.

[1] The Director of Naval Construction, William Henry White, provided two design options, both modified versions of the basic Majestic form.

Both variants had a main battery of four 12-inch guns, belt armour that was 8 inches (203 mm) thick, and a top speed of 18 knots (33 km/h; 21 mph).

[9][10] The Formidable-class ships were powered by a pair of 3-cylinder triple-expansion engines that drove two inward-turning screws, with steam provided by twenty Belleville boilers.

The boilers proved to be troublesome in service, with all three members of the class experiencing problems with them, particularly as the ships aged, between 1909 and 1914.

The guns had a muzzle velocity of 2,562 to 2,573 feet per second (781 to 784 m/s), and they were capable of penetrating 12 inches of Krupp armour at a range of 4,800 yards (4,400 m).

Throughout their peacetime careers, the ships were repeatedly overhauled and had minor modifications carried out, including alterations to their light armament, addition of searchlights, and installation of improved fire-control and wireless systems.

[19][20] Shortly after midnight on 1 January 1915, Formidable was torpedoed and sunk by the German U-boat U-24 off Portland Bill while on patrol in the Channel.

[21][22] The following month, Irresistible was sent to the eastern Mediterranean Sea to join the Dardanelles Campaign against the Ottoman Empire.

She took part in repeated, failed attacks on the Ottoman defences guarding the entrance to the Dardanelles over the course of February and March.

During a major Anglo-French attack on the coastal fortifications on 18 March, Irresistible struck a naval mine and sank, though most of her crew was evacuated; around 150 men were killed in her final battle.

[23] Implacable was ordered to join the Dardanelles campaign in March 1915, and she arrived shortly after the loss of Irresistible.

On the day of the attack, 25 April, the ship steamed off X Beach at Cape Helles, bombarding Ottoman defences as men went ashore.

In recognition of the critical support she had provided the troops as they attacked Ottoman positions, they named the landing site "Implacable Beach".