Attacks on pay and pensions are likely to result in strike action, warned Unison leader Dave Prentis.

Speaking at Unison’s annual health conference in Brighton, Mr Prentis said: “Our members will not acquiesce in a pay freeze. If our members vote for action I will lead that action until we win.”

He praised the government’s former announcement about the NHS becoming the preferred provider, but said: “If Gordon Brown really thinks that… is enough to secure our support he’d better think again. Our members won’t vote for privatisation.”

He added: “Unless we get policies that appeal to public service workers they’re not going to vote Labour.”

Mr Prentis afterwards told HSJ that the union was potentially prepared to protect “the pay and pensions issue interlinked” through industrial action.

He said that, during the general election campaign, Unison will continue to exert pressure on the government , which has faced criticism by the Conservatives in recent weeks for showing “weakness” towards unions such as Unite.

Mr Prentis described the preferred provider policy as a “major victory”, despite evidence that it is being widely ignored and is not supported by other cabinet members.

He said: “It’s our job now to make that improvement stick.”

Earlier that day, health secretary Andy Burnham had tried to reassure delegates that the policy was clear.

He said: “We’ve put a lot of time into making the preferred provider policy clear and we issued guidance with the help of the social partnership forum to the NHS just three to four months ago and it sets out in detail how it should be implemented.

“I’m passionate about good NHS provision and would expect if there are examples [of it not happening], we’d take these examples and look at the specifics.”

Asked about high NHS manager salaries, he said, against a chorus of jeers: “Let’s remember the NHS is a expanding system and it needs good management. Hospitals and PCTs are some of the most complex organisations that we have.

“But we have set a target for the reduction of management costs by 30 per cent… because we want to protect people on the frontline. It won’t be easy, but that’s the commitment we’ve made.”