Trade unions representing millions of public sector workers announced on Wednesday they would hold a strike ballot over pension reforms, raising the threat of mass walkouts later this year.

Unison, the biggest public sector union, said it would ballot 1.1 million of its members, while Unite and the Fire Brigades Union -- representing firefighters -- also said they would be putting strike action to a vote.

Unison did not join a nationwide strike over pensions in June, saying it wanted to keep negotiating over the reforms, but General Secretary Dave Prentis said he had now alerted 9,000 separate employers' groups to the possibility of industrial action.

"We've been patient, we've co-operated, but there comes a time when we say enough is enough because if we don't, they'll be back for more," he told delegates at the annual Trades Union Congress (TUC) in London.

He added: "A ballot unprecedented in scale will cover over a million workers in health, local government, school, further education, higher education, police, the voluntary sector and the environment.

"It's a decision we don't take lightly and the stakes are high, higher than ever before, but now is the time to make our stand. It will be hard, we'll be vilified, attacked, set against each other, but we must stay strong and united."

Prentis received a standing ovation from TUC delegates for his announcement, which will be followed by a meeting of union leaders later Wednesday to discuss dates for ballots and for the strike action itself.

The unions are protesting against reforms to raise the public sector retirement age by up to six years to 66, increase employee contributions by 50 percent in some cases, and replace final salary pensions with one based on average career earnings.

Employment Minister Chris Grayling said he regretted the unions' move but insisted there was no option but to reform public sector pensions.

"The truth is that there is a huge gulf between the pensions available today in the public sector and those available in the private sector," he told Sky News.

"It is simply not realistic to believe that there can be no change, particularly at a time when people are living longer and so the cost of pensions is rising and rising."