Intended as a shore bombardment vessel, M20's primary armament was a single 9.2 inch Mk VI gun removed from the Edgar-class cruiser HMS Gibraltar.

She was equipped with a four-shaft Bolinder two-cylinder semi-diesel engine with 640 horsepower that allowed a top speed of eleven knots.

HMS M20 ordered in March, 1915, as part of the War Emergency Programme of ship construction.

She was laid down at the Sir Raylton Dixon & Co. Ltd shipyard at Govan in March 1915, launched on 11 May 1915, and completed in July 1915.

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