Image copyright PA Image caption HMS Astute is at sea conducting trials

Britain's fleet of seven "attack" submarines are not on active operations, the BBC understands.

Normally at least one of the ships, which carry torpedoes and Tomahawk cruise missiles, is on operations.

But the one vessel currently at sea - HMS Astute - is conducting trials.

The Ministry of Defence said it would not comment on specific operations - but a source insisted that the Royal Navy had submarines that were "operationally capable and ready".

'Teething problems'

The BBC understands the Royal Navy is struggling to maintain the four ageing Trafalgar-class submarines.

The three newer Astute-class submarines have suffered "teething problems".

The Sun reported that five of the seven submarines were having refits or maintenance after breaking down.

The paper said another was being repaired after crashing into a tanker, while HMS Astute was at sea but not on missions.

A Royal Navy spokesman said it did not comment on "specific submarine operations", but added: "Britain has a world-class fleet.

"The Royal Navy continues to meet all of its operational tasking, deploying globally on operations and protecting our national interests as Britain steps up around the world."

The problems do not affect the Vanguard-class submarines, which carry the Trident nuclear system.