Ambulance crews have expressed concern at the lack of experience of stand-in firefighters of coping with emergencies during fire strikes.
A secret memo reveals they have been told by bosses not to try to rescue victims from car crashes or fires if they arrive at call-outs before the temporary crews.
It comes as 5,500 firefighters protesting at facing dismissal and re-employment on new shifts plan an eight-hour strike on Monday and a 47-hour stoppage on Friday that will include Bonfire Night.
Stand-in crews from private contractor AssetCo, employed by the London Fire Brigade to provide cover during the walkouts, were criticised for their delayed response to a house fire in Enfield during the first strike last Saturday.
Now brigade chiefs face further pressure after the memo warned that the temporary staff were likely to take longer to arrive at incidents and have little experience of tacking emergencies. The AssetCo staff are understood to include pub bouncers and security guards.
The London Ambulance Service today confirmed it had warned staff about the impact of the strikes after the memo was passed to LBC radio.
An ambulance service spokesman said: "We sent out an internal bulletin ahead of the planned industrial action by the Fire Brigades Union to make staff aware of the details and timings of the strikes.
"We reminded our staff not to carry out any duties that they're not trained for or normally expected to do. The advice given to staff is very similar to that given to them during the last fire dispute in 2002.
"We are working with the London Fire Brigade and our other partner agencies to ensure that we maintain our usual high standards of patient care."
A London Fire Brigade spokesman said: "The contingency fire cover provided for Londoners can't replace or replicate our day-to-day service, but staff have received professional training and will be available when Londoners dial 999 on Monday.
"If the FBU choose to go on strike, we have both a legal and moral responsibility to provide cover to protect people."
Today it emerged that eight London firefighters live overseas and commute to work, while 26 live in Wales, three in Northern Ireland and two in Scotland.
In addition, 2,052 live in inner London, 1,049 in outer London and 2,700 elsewhere in England.
Critics claim this makes a mockery of the "London weighting" of their pay.
Brigade chiefs are trying to force firefighters to work longer day shifts to increase fire prevention work.
A YouGov opinion poll today found 73 per cent of Britons believed the firefighters were wrong to strike on Bonfire Night.