Members voted overwhelmingly for action in a ballot that closed on 15 May. They will take strike action on 26 May over a number of pay issues, including a move from weekly to monthly pay.
Picket lines will be in place from 6am till 6pm at Hyde Park, Kensington Gardens and St James Park. There will also be a protest demonstration outside Downing Street from 10.30am.
The strike will affect the post-bank holiday cleanup. It is also one day before the State Opening of Parliament, meaning the Queen will pass unfinished bedding outside Buckingham Palace, the union said.
Gary Carter, GMB regional officer, said: "Employment relations have been good down the years, but since OCS took over the contract in August 2014 staff morale has rapidly deteriorated because of repeated attempts to worsen their pay and conditions.
"Now OCS wants to hold back two week’s pay our members have already earned which will put these lower paid staff into debt. The management's action will cause unnecessary financial hardship.
"The staff and gardeners are proud of the work they do which is admired by Londoners and by visitors from all around the world. They deserve to be treated with decency and respect and to earn a reasonable living. The Royal Parks should also not sit back and let this happen."
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