A total of 17 union general secretaries have called on the government to rule out implementing spending cuts.
The general secretary of the Trades Union Congress, Frances O'Grady, said: "Government departments have been tasked with finding efficiency savings.
"After 12 years of cutting services to the bone, there is nothing left to trim.
"When the Conservatives say efficiency savings this is usually code for cuts."
It is expected that Liz Truss and Kwasi Kwarteng will write to government departments to find efficiencies and save money.
Rising inflation and a lack of budget increases mean the public sector faces a real-term drop in financial backing.
Unions are already at loggerheads with public bodies over pay rises.
Many want a pay bump of around 10%, while offers tend to be around 2%, leading to industrial action in some cases.
In their letter, the 17 union bosses said: "Prime minister – you promised in your leadership election campaign that there would be no reduction in public spending.
"But according to reports, another wave of crippling austerity could be on its way in November in order to fund tax cuts for the super-rich.
"This would be an act of national vandalism and a huge betrayal of the British people.
"We therefore seek an urgent meeting with you and the chancellor, and a cast-iron assurance that you will not make further real-terms cuts to public services – now or in the future.
"Frontline services are already at breaking point. They must not be sacrificed to make the top 1% even richer.
"Unions will not sit by and allow the government to impoverish public services and the amazing staff who deliver them.
"We won't allow the social fabric of this country to be destroyed."
The letter was signed by
- Frances O'Grady - General Secretary, TUC
- Christina McAnea - General Secretary, UNISON
- Sharon Graham – General Secretary, Unite
- Gary Smith - General Secretary, GMB
- Kevin Courtney, Mary Bousted - Joint General Secretaries, NEU
- Patrick Roach - General Secretary, NASUWT
- Paul Whiteman - General Secretary, NAHT
- Gill Walton - Chief Executive, RCM
- Matt Wrack – General Secretary, FBU
- Mark Serwotka - General Secretary, PCS
- Mike Clancy - General Secretary, Prospect
- Dave Penman - General Secretary, FDA
- Steve Gillan - General Secretary, POA
- Iain Lawrence – General Secretary, NAPO
- Roy Rickhuss - General Secretary, Community
- Karen Middleton – Chief Executive, CSP
- Mark Sargeant – Acting General Secretary, RCP
- Annette Mansell-Green - Head of Employment Rights, BDA
- Paul Donaldson – General Secretary, HCSA