Protests are to be held across the country against the Government's 1% pay cap on civil servants.
Members of the Public and Commercial Service union will demonstrate on Friday outside buildings including tax offices, town halls, the DVLA in Swansea and the Treasury in London.
The union said that since 2010, a two-year wage freeze, followed by a 1% pay cap and changes to pensions, had hit living standards.
The union said average civil service pay fell by between £2,000 and £3,500 in real terms between 2010 and 2016.
The Government's plan to keep the 1% cap in place until 2020 "seriously undermines" the Prime Minister's claim that she wants to help "just managing" families, the union said.
General secretary Mark Serwotka said: "Brexit will put extra strain on a civil service already struggling to cope and to add insult to injury, dedicated public servants face three more years of pay cuts.
"Theresa May promised to help 'just managing' families and has the opportunity to start with her Government's own employees who are unable to make ends meet."