HMS Prince of Wales has left Portsmouth a day late after postponing its scheduled departure at the last minute.
The Royal Navy carrier, which was due to sail on Sunday, is replacing HMS Queen Elizabeth on a Nato exercise after it broke down a week earlier.
Retired Rear Admiral Chris Parry said the fleet's carriers were getting a "reputation for not being reliable".
He said he suspected there had been delays in getting the vessel's systems and equipment ready after maintenance.
HMS Prince of Wales is replacing HMS Queen Elizabeth in Exercise Steadfast Defender off Norway - the largest Nato exercise since the Cold War.
Its crew and base workers had been preparing the £3bn carrier to take over its sister ship's role, but its departure was postponed for unspecified reasons and it remained in Portsmouth overnight.
Mr Parry said: "The carriers are getting a reputation for not being reliable.
"They are big, complex bits of machinery, and the joy of having both of them is that the second one can step into the breach at a moment's notice - that moment has been a bit longer than we would have hoped.
"Prince of Wales has been in maintenance so I suspect there have been some delays associated with getting equipment and systems commissioned and ready for service."
Last Sunday, HMS Queen Elizabeth cancelled its deployment to the exercise off Norway at the last minute, after an "issue" was found with a coupling on the starboard propeller shaft.
In 2022, HMS Prince of Wales broke down off the Isle of Wight, when it also suffered a malfunction with a coupling on its starboard propeller.
The Ministry of Defence previously said the issues were "separate and not linked".
HMS Prince of Wales was taken to Rosyth, Fife, to undergo repairs which took nine months.
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