Nuclear-powered submarine HMS Torbay, based at Devonport in Plymouth, has a blue colour scheme in tests to find out the best colour for future vessels.

During their 105 years as part of the navy's fleet, most submarines have been painted black.

Black paintwork created a "harder outline" and was more easily spotted, a Ministry of Defence spokesman said.

A Ministry of Defence spokesman said: "This is an ongoing process of improvement, and if that means submarines will be less visible to the eye as a result, then all the better."

Submarines have been painted other colours in the past: two of the fleet were painted sandy-brown and green-black at the time of the first Gulf War.