Workers at homeless charity St Mungo’s will launch a month-long strike on Tuesday in a dispute over pay.
Members of Unite will mount picket lines outside offices in London, Brighton, Bristol and Oxford.
The union said the walkout was over a “pitiful” pay offer of 2.25 per cent.
Unite general secretary, Sharon Graham said: “Charity workers who should be on the streets helping the homeless have reached breaking point.
“The workers are now taking a stand.
“Instead of seizing the initiative to end the dispute, management’s decision to offer a pitiful 2.25% has spectacularly backfired.
“Now St Mungo’s faces a month-long strike and the workers have Unite’s total support.
“The pitiful pay offer has just made everyone in the union angrier.
“St Mungo’s have the answer in their own hands.
“Make Unite members a decent pay offer.
“Their indifference to the financial pressures facing their own staff is quite frankly astonishing.”
Emma Haddad, chief executive of St Mungo’s, said: “Our latest offer, combined with the annual pay rise proposed by the National Joint Council, would have meant a pay rise of at least 10% for those colleagues on the lowest salaries.
“This is what Unite has been asking for but voted against it.
“After all our efforts to find a solution to this dispute, a four-week strike is unprecedented and disproportionate.
“It will impact vulnerable people at risk of or recovering from homelessness.
“My door remains open to Unite, every day during the strike.”
