Antoine Delafargue, 34, and Michael de Lagarde, 35, set out from Plymouth in southwest England on Friday in a three-tonne, six-metre-long (20-foot-long) submarine.

The two men had hoped to complete the 250-kilometre (150-mile) crossing to Saint-Malo in northwest France within a week, each pedalling for up to 12 hours a day — the equivalent of a stage of the Tour de France every day.

But within hours of being submerged they were forced to return to port after a vessel tracking the submarine lost the use of its sonar, Gael Brelet, a spokesman for the Pilot Fish Project, told AFP.

On their return to Plymouth the sailors also noticed another problem.

Brelet said the submarine capsule showed “dangerous levels of carbon monoxide” but tests later showed the fuel cell was, in fact, leaking hydrogen.

Citing safety reasons, Delafargue and Lagarde decided to call off the mission and try again next year, Brelet added.

On the project’s website, it explains that, as well as crossing the stretch of sea, the two men aimed to film the ocean bed and promote energy conservation.

“We want to wake people up from their power and gas guzzling dreams,” they said in a statement.

Delafargue told British media he had funded the project himself at a cost of some £100,000 (118,000 euros, $130,000).

“I’d always wanted to build a submarine since I was a kid,” he said.

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