London Fire Brigade (LFB) has pledged to respond to 999 calls during Saturday's eight-hour strike.
Fire crews will walk out from 1000 BST in protest at new contracts that include changes to shift times.
LFB said contingency plans would provide fire cover for every area of the city but admitted a fire engine may not be sent to less urgent incidents.
The Fire Brigades Union (FBU) said 79% of members backed the action with another strike planned for 1 November.
LFB said up to 27 fire engines would be placed at strategic locations across London and was a "significant improvement" on the use of military green goddesses during the last strike.
They will be sent to a wide range of emergencies including fires in people's homes, vehicle fires, road accidents and collapsed buildings, it said.
The dispute centres on plans to scrap current rotas and force staff to sign new contracts or face the sack.