Up to 3,000 troops have been put on standby to run any prison hit by industrial action as a result of an imminent decision on whether to privatise some of Britain's biggest jails.

The justice secretary, Ken Clarke, is pressing Downing Street for approval to announce the winning bidders to run HMP Birmingham, HMP Buckley Hall in Rochdale, and a new "super-sized jail" on a site next to HMP Featherstone in Wolverhampton. The announcement is due before the end of the week.

The Prison Officers Association, which has a two-year-old mandate to take industrial action, up to and including strike action, in the event of any prison being privatised, is urging prison staff to hold gate meetings at the start of their shift the day after any such announcement, to discuss a strict work to rule.

Feelings are particularly strong at the 1,450-capacity Birmingham prison, which Jack Straw ordered to be market-tested when he was justice secretary after it was branded a "failing jail". Staff say they feel they worked hard to bring it up to a higher standard and don't want to see a private company profit from their hard work.

Colin Moses, POA chairman, said more than 500 prison officers went on the weekend's TUC anti-cuts march in London: "That is a massive contingent for us. It was the first time that we have marched alongside other trade unions."

Ministers got the agreement of the Ministry of Defence to train 2,000 to 3,000 regular soldiers in formal jailcraft earlier this year after studying the first national strike by the POA in 2007 when 90% of prison officers walked out for 12 hours. A reserve power to ban prison strikes was reintroduced the next year.

The army used to train its soldiers in staffing a jail as a matter of routine during the 1980s and 1990s but withdrew from the agreement 10 years ago in the face of the pressures of Iraq and other campaigns. The prisons minister, Crispin Blunt, a former army captain, got MoD approval a few months ago for up to 3,000 servicemen and women to be trained in prison craft such as how to lock down a wing, moving inmates around the prison safely, and understanding fire risks.

The numbers of soldiers who have been trained means that they could not provide cover for walkouts at more than a handful of prisons.

It is understood those involved are drawn mainly from the army's only prison, the military corrective training centre at Colchester, and elements of the RAF Regiment, which guards air force bases.

Although the prison service has also been talking to the police about contingency plans it is reluctant to get involved.

A Ministry of Justice spokesperson said: "Our aim is to avoid any form of industrial action and we will continue to talk to the unions to do everything possible to achieve this. However, given the risks and complexities involved in running prisons it is sensible and appropriate to review our contingency plans for dealing with widespread industrial action." She added that they were to ensure their contingency plans were "as strong as possible".

Bids from the public and private sectors were invited for the three prisons. It is believed that Serco and G4S, the largest UK private security companies, are involved in the bids.

Buckley Hall was one of the first jails to be privatised in 1994 but was taken back into the public sector in 2000. Featherstone 2 was initially earmarked as a Titan prison holding 2,500 inmates but has been scaled back to a "super-sized" 1,600 capacity.