The NHS workforce has grown 30% in a decade, Reform says
At least one million public sector jobs should be cut to ease the public debt, according to the Reform think tank.
The centre-right group says the NHS and the police service should be among the areas hit hardest, having previously enjoyed big workforce increases.
Reform also attacks government and Tory pledges to protect front-line jobs, saying those make up the majority of public sector posts and must be cut.
Unison union said the financial sector should pay more towards the deficit.
The three main political parties have suggested holding back public sector pay rises, but have not proposed widespread cuts to public sector jobs.
"They prefer to talk about making efficiency savings and trimming back office functions," says BBC political correspondent Ross Hawkins.
He says many politicians will fight shy of promising to make redundant the people taxpayers see delivering public services.
But Reform argues that "the public deficit cannot be reduced sufficiently without tackling the front line".
It says public sector employment costs need to fall by 15% - equivalent to one million jobs, and saving £27bn a year - to eliminate the government deficit in coming years.
Reform director Andrew Haldenby said: "Politicians have to be more honest with the public about what needs to happen if they are to reduce the deficit. Radical change won't happen if they see their role as the defenders of the status quo.
"The public sector workforce has to shrink to become as productive as the private sector. This must be an essential part of the plan to reduce the deficit that Alistair Darling should announce in his pre-Budget report."
Unison spokeswoman Mary Maguire said cutting public services was not the answer to the huge deficit.
"What Reform needs to do is go back to the drawing board and find a way to get the bankers, financial institutions and tax avoiders to pay their fair share of taxation," she said.
The chancellor delivers his pre-Budget report on Wednesday, when he is expected to confirm annual borrowing will top £175bn.
The government has promised to halve that figure within four years.
On Monday, Gordon Brown said overpaid public sector workers would be "named and shamed" in efforts to deliver more value for money in public services.
The prime minister said "efficiency savings" would help to save £12bn over four years - £3bn more than planned in the Budget.