A protest will be held on Monday over claims that a deal to end a 111-day dispute involving staff at the National Gallery is not being honoured.
Members of the Public and Commercial Services union working in the London gallery's visitor services staged a series of strikes in protest at the privatisation of their jobs.
The union claimed that more than a year after taking over the contract, private firm Securitas has "reneged" on a pledge over the London living wage and is creating a two-tier workforce by "driving down" pay and conditions.
PCS general secretary Mark Serwotka said: "The only thing Securitas is living up to is our prediction about what this sell-off would mean.
"It is outrageous that more than a year on, the company is refusing to honour its commitment on the living wage, and is creating a two-tier workforce and driving down pay and conditions."
The protest will be held outside the gallery in Trafalgar Square.