Three London fire engines have been unavailable for use because they were left "outside and unattended" during strikes, the Fire Brigades Union said.
The FBU said it was "shocking" that the engines now needed repairs before they could be returned to service.
But London Fire Brigade said there was "no delay or issue", and its fleet of 27 engines was back at fire stations.
All LFB fire engines were removed from service in October during a dispute over duty hours and contracts.
That row has since been resolved with the FBU voting in favour of 10.5-hour day shifts and 13.5-hour night shifts.
'Look in the mirror'
A private firm, AssetCo, holds the contract for maintenance.
In a letter to London fire commissioner Ron Dobson, FBU general secretary Matt Wrack said: "Three of the fire engines have still not been returned because they were left outside, unused and unattended, and now require repair.
"I find it rather shocking that they are now not in a usable condition, when they have been, since October, in the care of the company which is supposed to maintain London's fire engines."
A London Fire Brigade spokesman said: "The 27 fire engines affected by the strike are back at fire stations.
"There was no delay or issue with AssetCo relating to their return.
"Instead of accusing the brigade of delaying the return of the engines, the union should take a good long look in the mirror and ask themselves why they failed to call off the action for four weeks after agreement over shift pattern changes had been reached so that we could return the engines."