Two Frenchmen who tried to pedal their way across the Channel in a man-powered submarine had to abandon ship after sonar woes and a gas leak, organisers confirmed on Monday.

French engineers Antoine Delafargue, 33, and Michaël de Lagarde, 36, had set off from Plymouth on Friday afternoon and were due to arrive in Saint Malo in western France 250 kilometres (150 miles) away on August 12.

They claimed to be manning the "smallest long-range autonomous submarine ever built, and the first human powered one".