Cleaners and maintenance workers at the British Museum are to stage a dramatic strike today just before a Shakespeare event.
They'll walk out for three hours in a dispute over plans to hand their work to a private company.
PCS and Unite said they fear their pay and conditions will be cut.
The two unions, representing about 50 workers, believe senior managers will soon confirm that privateers will take over from April.
Today's strike will be between 7am and 10am, ending just an hour before the five-hour Shakespeare's London: Bard, bears and boards event begins.
There are plans for more action next Monday when there is a museum members' event in the evening.
PCS general secretary Mark Serwotka said: "With the cost of living soaring the last thing these low-paid workers need is a private company taking over and cutting their wages and working conditions to boost profits and line shareholders' pockets.
"Cleaners and other museum staff work incredibly hard to maintain this prestigious and popular cultural attraction and we know this is appreciated by the millions of visitors the museum attracts every year.
"Introducing the profit motive is not only unnecessary, it risks undermining these important services to the public."
Unite regional officer Carolyn Simpson said: "We will not stand idly by while our members and this vital museum service is sold-off to an external provider.
"The museum's standing as a world-class heritage site is in danger of becoming second class."