London Underground has indicated that staff refusing to work during the 48-hour strike starting at 9am next Tuesday could be sacked. However, the RMT general secretary, Bob Crow, said his union would support members who felt they were not able to work safely.
Mr Crow warned that any attempt to force workers to run trains in conditions they believed to be unsafe could be met by industrial action.
The health and safety executive is likely to declare that the railways and underground can operate without fire officer cover but Mr Crow said tube bosses had not yet convinced RMT safety staff that they could provide a safe workplace.
Government sources have said secondary action by rail and tube unions would not be tolerated, which means any action by Aslef or the RMT would result in a court injunction.
However, Mr Crow told Radio 4's Today programme that his union would be pointing out that "under section 7 of the Health and Safety at Work Act, workers mustn't place themselves in danger". He added: "We will not be calling our members out on secondary action because we are not allowed to."
Mr Crow said: "If any company places our members in danger we will ballot for action, because we wouldn't be worth our salt as a trade union if we weren't prepared to give support to people to come to a safe workplace.
He raised doubts over whether soldiers using green goddess fire engines would be capable of dealing with an emergency on the railways or the London Underground.
"The firefighters give a superb service to our members and the travelling public and I don't believe the green goddesses can supply that kind of protection," he said. "There were 1,000 calls last year for the fire brigade to go to the London Underground.
"I've had the opportunity to see a green goddess and it's an absolute laughing stock. How a green goddess is going to get to a site on the underground, I don't know.
Tube management had hoped to close just 19 stations, but it is now expected that the overwhelming majority of underground services will not run next Tuesday and Wednesday.
Mr Crow said: "To close 19 tube stations is an absolute sop, because if a train breaks down, who is going to evacuate those people with breathing apparatus, because the fire brigade has got 20 breathing apparatuses for London and the green goddess has got two."
