The government is on course for an autumn showdown with the trade unions after Treasury chief secretary Danny Alexander insisted ministers would not back down over plans to reform public sector pensions.

Mr Alexander made clear that while the government was prepared to discuss the detailed implementation of the proposals, it was sticking to the broad principles that will see millions of workers having to pay more and work for longer.

Shadow chancellor Ed Balls urged the unions not to fall into the government’s “trap”, accusing ministers of deliberately engineering a confrontation so that they could blame the unions for the failing economy.

However, union leaders - who are threatening disruption on a scale not seen since the general strike of 1926 - said they had to defend their members’ interests in the face of the government’s onslaught.

Mr Alexander’s intervention came after business secretary Vince Cable had appeared to strike a more conciliatory tone, stressing the government’s commitment to negotiation, saying “reasonable people” should be able to reach a deal.

However, Mr Alexander made clear that any agreement had to be based on the proposals set out in the government-commissioned review of public sector pensions carried out by former Labour cabinet minister Lord Hutton.

“They are the basis on which we want to go forward and reform public service pensions, but of course in these discussions we need to look at the detail of how that works, about how these things are implemented,” he told Sky News’s Murnaghan programme yesterday.

“What we have to get to is a situation where, yes, people have to work a bit longer and contribute a bit more, as we have put forward, but that we maintain the quality of their pensions into the future.”

His comments infuriated union leaders, still angry over a speech he gave on Friday, which, they said, had threatened to derail long-running negotiations to resolve the dispute.

They said that a planned one-day strike on 30 June by up to 750,000 teachers and civil servants would almost certainly be followed by further mass action in the autumn unless ministers changed course.

“This is the final straw,” said Unison general secretary Dave Prentis.

“Having attacked their jobs and now to take away their pensions in the way this government is doing, it leaves them no alternative to say we’re going to have to take a stand.

“If we’re going to be treated with disdain in the way that we are being now, then we will move to an industrial action ballot. And it will be the biggest action since 1926 because up to 10 million people will be involved.”