Union leaders have threatened to walk away from talks over pensions reform after the Government detailed plans to require most public sector employees to work longer and pay more for less generous entitlements in retirement.
Chief Secretary to the Treasury Danny Alexander provoked fury by warning public sector workers it would be a "colossal mistake" to reject a deal that was the best they could hope for.
The reforms include increasing the general retirement age in the public sector from 60 to 66, moving from a final salary system to benefits based on career-average earnings and raising contributions by an average of 3.2%.
But Mr Alexander insisted that those on the lowest incomes would not have to pay any more and that low and middle earners would get roughly the same benefits as they do now.
The proposals, based on the recommendations of former Labour Cabinet minister Lord Hutton, come amid a threatened wave of industrial action starting with up to 750,000 teachers and civil servants going out on strike on June 30.
Unions chiefs responded angrily to Mr Alexander's intervention, accusing him of trying to sabotage negotiations by announcing details of the Government's position to the media. Unite union assistant general secretary Gail Cartmail said Mr Alexander's intervention was "tantamount to bombing the talks".
The GMB threatened to pull out of the negotiations altogether. Its national secretary for public services Brian Strutton said the Government appeared to have "already made its mind up on some of the matters we are negotiating on".
Describing the Government's position on contribution increases as "plain barking mad", he added: "I was convinced that the Government was reconsidering its position on that and thinking it through carefully. I'm worried now that they are not. If Danny Alexander is going to say 'Actually, we've made our minds up', then that is a show-stopper."
Labour leader Ed Miliband said, during a visit to Greenock in Scotland: "My advice to the Government is, instead of shouting from the rooftops, they should engage in proper dialogue and negotiation with the workforce. That's the way you sort things out, not shouting as they have been doing, not engaging in megaphone diplomacy."
But Downing Street said the Government was trying to have a "constructive dialogue" with the unions and pointed out that almost half - 44% - of public sector workers were non-members. Prime Minister David Cameron's spokesman said: "Clearly, we have a duty to speak to those people as well."