A petition calling on the Government to scrap the public sector pay cap and give health, council, police and schools staff a decent pay rise will be debated in Parliament on Monday.
Almost 150,000 people have signed the petition, launched in September by Unison, demanding an immediate pay rise for all public service staff, at least in line with inflation.
Ahead of the debate, Unison general secretary Dave Prentis has written to the Prime Minister to ask her why the Budget failed to address the "woeful" lack of pay rises for public service employees.
He said: "For more than seven years, public service workers have been getting steadily poorer as the gap between their wage rises and the cost of living has grown.
"As a result they've been left thousands of pounds a year worse off, which has had a profound impact on the finances of millions of families, and on local economies, as households with public service employees rein in their spending.
"Services too are suffering as many experienced staff from hospitals, local councils, schools and police forces, opt for less stressful, better paid jobs outside the public sector, and those same services struggle to attract sufficient numbers of new recruits.
"Even though there was talk in the Budget of more money for health workers, reports since suggest that NHS staff may have to pay for their own wage rise, a hardly acceptable state of affairs after years of pay pain.
"Each and every month, the value of public sector pay packets is in decline, and with inflation now 4%, the gap between what dedicated public servants are paid, and what they can afford, grows ever wider."
Mr Prentis said it looked as though the Government had no plan to restore public sector pay to levels before the financial crisis, or to find a way of stemming the steady flow of "disillusioned" public sector staff who are resigning.
The debate, in Westminster Hall, follows months of campaigning by unions for the controversial cap on public sector pay to end.