Staff employed by the British Medical Association (BMA) have voted to take industrial action in a row over pay.
The GMB said its members at the BMA, which is taking its own industrial action next month over doctors' pensions, backed action by 4-1.
GMB members have already rejected a pay offer the union said was worth 1.5% plus an additional 0.5% for high performers only.
Under a grading re-structuring, the offer was worth less than 1.29% for two-thirds of staff, said the GMB.
Union official Anna Meyer said: "We have asked the BMA to re-open negotiations to avert industrial action.
"The next step is for GMB officials and shop stewards to meet to consider the ballot result and to plan the industrial action if the dispute is not resolved.
"The pay offer from BMA fell far short both of the RPI inflation for the year and of the median increase for pay settlements, which stands at 3% for the three months to January 2012.
"This is double what the BMA offered and which GMB members rejected.
"Despite repeatedly criticising the Government for refusing to enter into serious negotiations on doctors' pensions, the BMA has stated this is its final offer. Ironically, the BMA says that the cost of supporting the final salary pension scheme for just under half of its staff is a key reason for suppressing pay."
The BMA announced yesterday that doctors will take action on June 21 in protest at the Government's controversial pension reforms.
BMA chief executive Tony Bourne said: "Whilst we respect the rights of staff to take action, we are disappointed by the outcome of the GMB ballot.
"We continue to believe that the pay offer made to our staff this year is a fair one given the challenging economic environment facing all employers, given the very attractive terms and conditions of employment afforded to BMA staff and given the 5% award paid to staff last year.
"The BMA provides secure employment and offers competitive salaries, as well as maintaining a generous pension scheme. This year, we have also made very substantial, additional contributions to the staff pension scheme."
The BMA said it has offered staff a general rise of 1.5% plus an 0.5% performance pot as part of the 2012/13 pay award.