Staff employed by the British Medical Association are going on strike in a row over pay, a day before doctors represented by the group take industrial action in a separate dispute over pensions.
Hundreds of members of the GMB who work for the BMA will walk out for 24 hours after rejecting a pay offer of 1.5%, with an extra 0.5% for high performers.
It will be the first strike by BMA staff in the 180 years the organisation has been in existence, said the GMB, whose members include regional officers, researchers, librarians, administrative staff and industrial relations officers.
Picket lines will be mounted outside BMA offices across the UK, including London, Belfast, Edinburgh, Leeds and Cardiff.
Anna Meyer of the GMB said: "We are very disappointed that there has been no response to our calls for more talks and for an improved offer. GMB members are striking reluctantly. The offer is well below inflation and the BMA can well afford to make an offer in line with inflation.
"What GMB members find difficult to accept is that the BMA itself is embarking on industrial action on Thursday over the Government's refusal to reopen negotiations in their dispute. The Government's austerity measures have just plunged us back into recession. Instead of paying off the deficit, it has actually increased."
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.
"In all but one of the last seven years, the BMA has increased salaries above the market rate, and in some years, significantly above this."
Doctors represented by the BMA will take industrial action on Thursday for the first time in almost 40 years in protest at the Government's controversial pension reforms.