Public sector workers have been warned that it would be a "colossal mistake" to spurn the Government's pensions deal and sacrifice the best offer they will be made "for years to come".

Against the threat of a wave of industrial action, Chief Secretary to the Treasury Danny Alexander urged rank-and-file union members to help "shape" the current reforms now or face "uncompromising" change later.

On BBC Breakfast, Mr Alexander said public sector workers will now need to work to the state pension age, which is due to rise to 66 by 2020.

He said: "The plain fact is that people are living much longer now than they were even 20-odd years ago and that's a fantastic thing for society.

"But we have to ensure that it is affordable in the context of our pensions, and we're asking people through the state pension age to work longer; in the private sector people are working longer and the same is going to need to happen in the public sector too.

"That means that people have to work to the state pension age, that's what we're proposing, that we will increase people's pension contributions over the next three years."

He said the debate over public sector pensions was "bedevilled by extremes" but there was a "measure of agreement in certain areas" in talks between union leaders and the Government.

GMB union national secretary for public services Brian Strutton said Mr Alexander's intervention could be a "show-stopper" for negotiations.

Mr Strutton told the BBC Radio 4 Today programme: "What he seems to be saying is the Government has already made its mind up on some of the matters we are negotiating on."

But ministers insist that the changes are necessary given spending cuts, rising life expectancy, and when most private sector workers do not enjoy such generous pensions.