Public sector workers including refuse collectors, midwives and teaching assistants face a real terms pay cut averaging around £4,000 by 2020 amid a Government wages cap, a union claims.
Research by the GMB found £1,400 of the figure could be attributed to additional inflation caused by Brexit.
The current pay policy affecting public sector employees is the most severe for decades, said the report.
Social workers will see a real terms pay cut of almost £13,000 between 2010 and 2020, £18,300 for staff nurses and £22,700 for midwives, said the union.
Pay rises in the public sector have been held below inflation since 2010, said the GMB.
National officer Rehana Azam said: "The gruelling salary squeeze Theresa May is presiding over is even more severe than under Margaret Thatcher and John Major, with many workers set to effectively lose tens of thousands of pounds by 2020.
"It's high time for public sector workers to be paid their dues, this decade long pay pinch needs to end.
"Nurses, teaching assistants, midwives, council staff and 999 call handlers are on the front line every single day but have had thousands of pounds pinched already and are facing further attacks on their living standards."