The Prime Minister sparked outrage after it was revealed he has tasked his Cabinet with cutting about 90,000 jobs.
Boris Johnson is understood to have told ministers on Thursday that the service should be slashed by a fifth.
Unions reacted with fury, with one leader warning that national strike action was “very much on the table”.
He has chosen to cause our cost-of-living crisis and is desperate to point the blame somewhere, and he has chosen to point the finger at hardworking PCS members who kept the country running throughout the pandemic
Mark Serwotka, PCS
The Public and Commercial Services union (PCS) is to hold an emergency meeting of its executive committee next week to discuss its response.
General secretary Mark Serwotka told the PA news wire that any job cuts would affect anyone relying on public services.
He said: “The Government complains about longer delays for passports and driving licences at the same time as sacking the people who are working so hard to clear the backlog.
“This is not about efficiency. This is about the Prime Minister trying to create a smokescreen to detract from his utter shambles of a Government.
“He has chosen to cause our cost-of-living crisis and is desperate to point the blame somewhere, and he has chosen to point the finger at hardworking PCS members who kept the country running throughout the pandemic.
“Our members will not be the scapegoats for a failing Government. We have our conference in 10 days’ time. Taking national strike action is very much on the table.”
Dave Penman, general secretary of the FDA, which represents senior civil servants, told PA: “The reason for the Civil Service’s expansion since 2016 isn’t because the Government loosened the purse strings. The Government needed civil servants to deal with the consequences of two unprecedented events: Brexit and the Covid pandemic.
“To govern is to choose and ultimately this Government can decide to cut the Civil Service back to 2016 levels, but it will also then have to choose what the reduced Civil Service will no longer have the capacity to do. Will they affect passports, borders or health?
“Without an accompanying strategy, these cuts appear more like a continuation of the Government’s Civil Service culture wars, or even worse, ill-thought out, rushed job slashes that won’t lead to a more cost-effective government.”
Mike Clancy, general secretary of the Prospect union, said the proposal represented “an outrageous act of vandalism on our public services.”
“Through Brexit, and then the pandemic, we have never been more reliant in peace time on our Civil Service,” he said.
“Our members are highly skilled and there is a real risk to Government delivery from losing their vital expertise.
“The big cuts to public services since 2010 have often proved an expensive error – these proposals risk doubling down on the mistake.”
TUC Deputy General Secretary Paul Nowak said: “The Government is yet again treating the Civil Service with contempt.
“Civil servants are key workers who keep this country running. They deliver vital services, collect taxes, help people back into employment, regulate medicines, negotiate trade deals and thousands of other things that bind society together.
“It is shameful that the Prime Minister is throwing them under a bus to distract from the Government’s failure to deal with the cost-of-living crisis.”
Jacob Rees-Mogg, Minister for Brexit Opportunities and Government Efficiency, defended the plan on Friday, saying the job cuts would bring numbers back to 2016 levels after extra staff were brought in to help deal with the pandemic and the “aftermath of Brexit”.
He told Sky News: “I know it sounds eye-catching but it’s just getting back to the Civil Service we had in 2016. Since then, we’ve had to take on people for specific tasks.
“So dealing with the aftermath of Brexit and dealing with Covid, so there’s been a reason for that increase, but we’re now trying to get back to normal.”
Mr Johnson made the demand during an away day with ministers in Stoke-on-Trent, with the Government coming under intense pressure to ease the pain of soaring prices.
Sources familiar with Boris Johnson’s Cabinet conversation said he told ministers to return the Civil Service to its 2016 levels in the coming years.
It was said its numbers had grown to 475,000 full-time equivalent jobs.
The Prime Minister told the Daily Mail newspaper: “We have got to cut the cost of Government to reduce the cost of living.”
He suggested the billions saved could be used for tax cuts, saying: “Every pound the Government pre-empts from the taxpayer is money they can spend on their own priorities, on their own lives.”
Sources did not deny that the sweeping cut to public jobs could be used for future tax reductions.
Ministers are expected to report back within a month with plans for achieving the cuts from their departments.
A Government spokeswoman said “the public rightly expect their Government to lead by example and run as efficiently as possible” as the nation faces rising costs.