Defence sources have dismissed as "political posturing" reports that soldiers will be deployed as strike-breakers.

The military has in the past been brought in to cover for strikers, most recently in the fire service, but has done so specifically to ensure public safety. There is no appetite for troops to cover for immigration workers or tube drivers, for example, as reports suggested.

One official source described the suggestion as "not realistic". Another said any proposal to bring in troops in areas not related to public safety but, for example, to prop up the economy would be entirely new territory.

The contingency plans, said to have been discussed by David Cameron and the Cabinet Office minister, Francis Maude, appear to have come as a surprise to the Ministry of Defence.

Officials pointed out that apart from the fire service, other groups that could have an impact on public safety, including prison officers and the police, were prevented from taking strike action.

Ministers appear to have been impressed by the way the military came in to help control entry into Olympic venues after the failure of the private security company G4S to recruit and train sufficient people. But this was an informal, entirely voluntary arrangement.

The Fire Brigades Union has warned the government that using soldiers as a replacement public sector workforce in the event of co-ordinated mass strikes risks politicising the army.

The FBU's general secretary, Matt Wrack, said reports of creating an auxiliary workforce to mitigate walkouts would not be welcomed by members of the armed forces who were also facing redundancies and spending cuts. "It starts to politicise the role of the armed forces in relation to political and industrial matters," said Wrack.

He added: "It's provocative, especially after they have made hundreds if not thousands of service people redundant, suffering the same austerity cuts as anyone else. I suspect armed forces personnel themselves may not be too happy about it."

The report in the Times of a government strike contingency strategy emerged as the annual Trades Union Congress in Brighton carried a motion calling on the union umbrella body to consider the practicalities of calling a general strike. A day earlier the TUC carried a motion calling for co-ordinated action over public sector pay restraint, a scenario viewed as much more likely than a general strike.

The TUC general secretary, Brendan Barber, who has played down the chances of a general strike, also warned that involving the armed forces in strike contingencies would politicise the army. "The government should concentrate on resolving issues through negotiation, not through the provocative use of troops as strike-breakers," he said. "It is hard to think that the armed services would be happy to used in this highly political way. It would mark the UK out from other similar advanced democracies."

Help provided by the armed forces in the event of emergencies and threats to public safety are governed by Military Aid to the Civil Authorities (Maca), a term that contains specific criteria relating to maintenance of law, order and public safety. They include the principle that military intervention should always be the last resort and that the need for the military to act is urgent. Maca includes disarming unexploded bombs, air-sea rescue and mountain rescue.

A spokesperson for the Prime Minister said: "Of course we have contingency plans in place. Clearly I'm not going to comment on specifics of our contingency plans but those plans have not changed. Those plans are designed to take action over a range of scenarios. But the main point here is we do not think strike action is appropriate: it won't benefit anyone, we are not intending to re-open negotiations on pay and pensions, which have already been settled."

A Ministry of Defence spokesman said: "National contingency arrangements are a matter for the Cabinet Office. Established procedures for the provision of military assistance to the civilian authorities exist, which are tried and tested and were employed during both the recent Olympic Games and in preparation for a possible fuel strike earlier this year. These have not changed."