The Government was on a collision course with millions of public sector workers today after union leaders warned of a series of strikes over pensions.

Leaders of 13 unions representing more than three million workers in the NHS, local government, civil service, fire service and other public sectors pledged to campaign together against controversial proposals to raise the retirement age from 60 to 65.

Dave Prentis, leader of Unison, issued a blunt warning that workers will take strike action to defend their pensions.

He told the TUC Congress in Brighton that ministers had underestimated the anger of workers and did not understand their outrage, which has been fuelled by big pensions rises for politicians and company directors.

"To those who think that the anger has subsided I say think again. I promise you this, we are stronger and more united than ever before and we will take strike action to defend our pensions."

Alan Johnson, the Trade and Industry Secretary, who listened to a string of attacks from speakers over the Government’s plans, told delegates he was confident that negotiation, which will resume next week, will resolve the dispute.

Mr Johnson said that the Government faced a "dilemma" over pensions because people were living much longer and it was no surprise that it was having to review pension arrangements for its own employees.

He conceded that the early approach taken by the Government was wrong and pledged to continue negotiating with the unions. But he added: "Whilst we do want to change the retirement at 60 ethos, this is in the context of preserving high quality, defined benefit, index-linked pension schemes.

"We want to make improvements to other elements of the schemes and introduce arrangements which give individuals a choice about when they retire, be it aged 60 or 65 or later.

"But we can only deliver this within a scheme that is capable of withstanding the demographic changes that are bound to have a radical effect on pension provision.

"I look forward to further discussions and if we face the facts together, I’m confident that our negotiations will succeed."

Delegates gave Mr Johnson only a few seconds of half-hearted applause. Mr Prentis said later that the Trade and Industry Secretary was "playing for time" although he stressed the unions were keen to achieve a negotiated settlement.

The Unison leader said he had never known as much anger among public sector workers over the planned changes to their pensions.

"They are up in arms about their pension rights and I don’t think the Government realises the strength of feeling."

Mr Prentis said he feared local government workers would be offered a worse pension scheme than other public sector employees, which could also fuel calls for industrial action.

Local authority employers were pressing for a deadline to resolve the despite, said Mr Prentis, but he warns that could trigger conflict sooner.

Mark Serwotka, general secretary of the Public and Commercial Services Union, said he had no doubt that civil servants would vote in their tens of thousands in favour of strikes if the Government did not back down.

He predicted that industrial action on such a scale would have a "significant" impact across the public sector and would also affect the relationship between the Government and its workers.

"If the Government imposes an increase in the pension age, industrial action is absolutely inevitable."

Shadow trade and industry secretary David Willetts said: "The cynicism of the way the Government has handled public sector pensions is extraordinary even by its own standards.

"Last year’s tough talk about public sector pensions reform swiftly disappeared when the prospect of strikes arose prior to the election. Local government pensions proposals were ditched when it seemed like they might rock the boat.

"Mr Johnson wants everyone to think that he’s playing the hard man to the TUC, but the truth is these switches will fatally undermine its credibility and authority if it tries to deliver any changes."

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