Looks to be an issue with keeping the Trafalgar class submarines online.
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.