All of the Royal Navy's most powerful warships are in port at the same time, the Ministry of Defence has said.
The six £1 billion Type 45 destroyers are docked in Portsmouth for reasons that include giving crews time with their families over the summer, according to an MoD spokesman.
Tom Sharpe, from the Directorate of Defence Communications, said the scenario was "unusual but not unprecedented" and the ships could be docked for "anything from a few weeks to a month".
He added it was not connected to the need for all six ships to be refitted with new engines, which broke down in the Persian Gulf because they were not designed for the hot water.
Mr Sharpe said: "It happens most Christmases. To have them docked for a sustained period over the summer is unusual but not unprecedented.
"They (the crew) have got to take leave and we've got manpower issues which are much publicised. It's got to be planned in advance.
"They spend so much time away and it's about harmony time - getting the work life balance right.
"You could argue if you are trying to rebalance harmony time, it does not look good but in reality it combines everyone getting away at the right time of year.
"It's just a coincidence, nothing to do with the engines."
He added no other parts of the fleet were being left without protection by the destroyers and US warships were carrying out that role for those on operation in the Gulf.
It comes after pictures emerged showing significant damage to the nuclear-powered submarine HMS Ambush which had collided with a merchant vessel, leading it to be recalled to Britain for repairs.
Earlier this month, First Sea Lord Admiral Sir Philip Jones told the House of Commons Defence Committee the innovative gas turbine engines driving destroyers "degraded catastrophically" in very warm seas.
The Ministry of Defence will have to pick up the full bill for the multimillion-pound refit.
Described as part of the backbone of the Royal Navy, Type 45 destroyers are the most powerful ships ever built for use by the British Armed Forces.
The first of six, HMS Daring, was commissioned in July 2009 and was followed by sister ships Dauntless, Diamond, Dragon and Defender.
The final, HMS Duncan, was commissioned in 2013.
A Ministry of Defence spokesman said: "This week the Royal Navy had over 8,500 personnel deployed on operations around the world and 34 ships at sea.
"All Type 45 destroyers are currently in port as they have either just returned from operations, or are about to be deployed, are conducting training or carrying out maintenance or are home for crew to take summer leave."