The health sector of Unite have rejected the government's "politically motivated" final offer on pensions, raising the possibility of more strikes.
In a strongly worded statement, Unite general secretary Len McCluskey says: "The government's attacks on public sector pensions are politically motivated, as part of an overall design to privatise the NHS, cut public services, break-up the national pay agreements, and disrupt legitimate trade union activities and organisation.
"Unite believes it is important to continue a campaign to maintain a fair and equitable system of public sector pensions and calls on ministers to enter into real, genuine and meaningful negotiations on the future of NHS pensions and public sector pensions."
The move by Britain's second largest union, which represents over 100,000 health workers, comes as the British Medical Association (BMA) prepare to survey 130,000 doctors and medical students over a possible ballot.
The doctors' trade union, which last took industrial action 30 years ago over junior doctors' working conditions, did not take part in the huge strike by public sector workers in November 2011.
Since 1975, industrial action has been raised as a possibility – but disputes have not reached the point where a ballot was necessary.
Dr Hamish Meldrum, chairman of the BMA Council, said: "The BMA, along with the other unions, has not accepted the offer. That, quite rightly, is for our members to help decide. Throughout intensive negotiations, we repeatedly pointed out that the NHS pension was radically overhauled only three years ago, and is actually delivering a positive cashflow to the Treasury."
NASUWT, the teachers’ union, is also meeting tomorrow to consider the pension proposals, amid speculation that they may reject them.
Unite has said it plans to meet again on the 11 January to discuss further action.
A Health Department spokesman said Unite's decision was "disappointing", adding: "The proposed new NHS Pension Scheme is a good deal - it is fair to the NHS workforce, it is fair to the taxpayer and makes public service pensions affordable and sustainable.
"The improved offer on public service pensions protects all those within 10 years of their pension age from any further change. It means the nurses and doctors who dedicate their life to treating us will continue to receive the best quality pensions available in this country.
"While most workers will still have to work longer and pay more, most low and middle earners working a full career will receive pension benefits at least as good, if not better, than they get now.
"The Government has made clear this is our final position on the main elements of scheme design, which all the health unions, except Unite, agreed in December to take to their members.
"Unite represents approximately 7% of the NHS workforce - it is important to remember that it is the decision of the unions representing the majority of staff which will determine the final NHS pension deal. Further discussions are being held to work on the remaining details."