The Unite union said the 72-hour strike follows a “derisory” offer of a 1% pay increase plus a non-consolidated (one off) 1% payment.

Unite members are seeking an increase of 3.5% the retail price index (RPI) as of April 2018 (the annual pay date) or a flat rate of a £1,100 increase for all workers.

More than 400 of the union’s members will take part in the strike from December 11.

According to Unite, Shelter’s workforce has suffered a real terms pay cut of 11% since 2010 because of a series of below inflation pay increases. Shelter recorded a surplus of £1 million last year and has £15.7m in reserves.

In testimonies provided by Unite members, Shelter staff said they felt “undervalued and underappreciated” at a time when they are “working above and beyond capacity every day”.

Another said: “I am very worried about the implications of this on my own financial obligations and responsibilities. We are here to support people on low incomes, but Shelter staff are now facing this themselves.”

“Shelter’s vision of a safe secure affordable home for everyone is a bit meaningless considering I am struggling to pay my mortgage every month,” one member of staff added.

Unite said the strikes could seriously affect the organisation’s ability to offer advice and support to the homeless and people in precarious housing in the pre-Christmas period.

If the dispute is not resolved, Unite will hold further strike action in the New Year.

Unite regional office Peter Storey said: “Our members are not greedy. They are not looking to get richer, they are just trying to keep their heads above water.

“Unite’s members at Shelter have faced a year on year pay squeeze since the beginning of the decade and this year’s derisory pay offer was the final straw that broke the camel’s back.

“It is entirely unfair that the wages of Shelter workers have been declining at the same time that the charity’s reserves have been increasing.

“The management at Shelter should stop trying to pressurise our members not to go on strike and instead enter into meaningful negotiations to resolve the dispute.”

Shelter Scotland has been contacted for comment.