All six of the Royal Navy's Type 45 destroyers are currently docked in port, at a time of heightened tensions with Russia.
The warships, equipped with guided missiles have been plagued by engine issues since they were launched, with vessels now undergoing a power improvement project (PIP).
Announced in 2018, the Ministry of Defence (MOD) said it is intended to "provide a robust solution to the power and propulsion issues observed in Type 45".
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The Defence Select Committee warned in December 2021 that the "low availability of the UK's Type 45 destroyers and recognised issues in their propulsion systems are a major cause for concern."
HMS Dragon was pictured entering Portsmouth on Monday, joining HMS Defender, Diamond, and Duncan alongside.
HMS Dauntless and Daring are docked in Birkenhead, receiving modification as part of the PIP.
It is understood that HMS Defender and Diamond are "at notice to sail", but remain alongside.
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Tom Sharpe, a former Royal Navy Commander, told the PA news agency that availability is "unlikely to improve any time soon".
"This is about alliances versus redundancy.
"On the one hand, that we have allies we can plan with and then call on to prove surface-to-air coverage is good.
"On the other hand, that our fleet is now stretched so thin that a major set-piece deployment such as CSG21 reduces subsequent availability to zero, is clearly bad.
"It's also unlikely to improve any time soon, despite pledges to increase spending and hull numbers.
"Playing tunes on what constitutes availability, or worse, resorting to 'if there is a war, we could surge' doesn't ameliorate just how taut things are and how hard the Navy is being made to work to manage it.”
Vice Admiral Sir Chris Gardner told the Defence Select Committee in November 2021 that all six ships would have their defects repaired by 2028.
He also confirmed that HMS Diamond, while deployed with HMS Queen Elizabeth in July 2021, suffered "a failure of one of her gas turbines… and we had to replace it".
The Chairman of the Defence Select Committee, Tobias Ellwood, told the PA news agency: "The inability for any of our six Type 45 Destroyers to put to sea is a reflection of just how small our surface fleet has become.
"Our world is becoming more dangerous not less. Operational taskings for our surface fleet are increasing not decreasing.
"As our recent Defence Select Committee report concluded, our Navy will soon be too small to defend our interests and deal with emerging threats.
"We must seriously consider doubling the size of our maritime forces if our hard power is to stay ahead of the emerging challenges to our security and access to international waters that are critical for our economy."
A Royal Navy spokesperson said: "The Royal Navy is meeting all of its operational requirements, managing ships across the fleet at different levels of readiness in the usual way.
"The Type 45 is a world-class Destroyer and is a key part of the Navy's Carrier Strike capability, which makes an enormous contribution to the defence of the UK and our partners."