Unions have raised fears a review of the number of fire engines operating in London could lead to 500 job cuts.
An investigation into the feasibility of removing 27 appliances from the fleet was ordered by London's fire authority chairman Brian Coleman.
It comes after the vehicles were handed over for use by private contractors AssetCo during the recent strikes.
The Fire Brigades' Union (FBU) described the review as a threat to front-line firefighting services.
Mr Coleman said he wanted the fire commissioner to examine whether the brigade needed all 27 of the fire appliances removed from stations during the current industrial action to be returned and whether there is an over-supply of appliances.
A union spokesman said it vindicated their claims that cuts were at the heart of the firefighters' dispute and could result in up to 500 posts being axed.
Ben Sprung, of the FBU, said: "Coleman has denied our dispute had anything to do with cuts in the service for Londoners.
"This proves that has been the agenda all along. He seems willing to put his vendetta against firefighters above the safety of London."
'Rigorous risk assessment'
The union is currently embroiled in a row with LFB over claims its members face the sack unless they agree to sign new contracts in which their shift patterns are changed.
A 48-hour strike planned for Bonfire Night was called off the day before when the FBU urged the London Fire Brigade (LFB) to put back its deadline on the contracts and make a decision in the new year, rather than later this month as planned.
The original compromise agreement of an 11-hour day shift and a 13-hour nightshift will be discussed in more detail during talks on 16 November.
Mr Coleman made the last-minute emergency amendment at Monday's London Fire and Emergency Planning Authority budget committee.
But Matt Wrack, FBU general secretary said: "To change fire cover, the fire authority would have to go through a rigorous risk assessment process. This has not been done."