Boris Johnson has ordered ministers to come up with plans to cut more than 90,000 civil service jobs in order to free up billions of pounds for tax cuts.

During an away-day with cabinet ministers in Stoke-on-Trent, the prime minister asked them to report back within a month on how they can reduce the size of their departmental workforces to 2016 levels.

That would imply a reduction of about a fifth in the 475,000-strong workforce. Details of the plan would be set out in due course, the government said. It would be expected to save about £3.5bn a year.

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The announcement was described as “either another headline-grabbing stunt or a reckless slash-and-burn to public services” by the head of the civil servants’ union, warning that the Passport Office or Department of Health and Social Care might suffer.

It comes as Mr Johnson faces pressure to do more to address the cost of living crisis, which has seen inflation soar to its highest level in three decades – with Tory MPs pressing for tax cuts and Labour accusing him of being “bereft of ideas”.

The PM told the Daily Mail, which first reported the planned cuts, that the civil service had become “swollen” during the pandemic.

He added: “Every pound the government pre-empts from the taxpayer is money they can spend on their own priorities, on their own lives.”

The announcement appears to flesh out a comment from the PM during the Queen’s Speech debate earlier this week, when he spoke about needing to “cut the cost of government”.

ITV News reported that the PM and chancellor met on Monday to start drawing up the plan, which would include a ban on vacancies being filled without special permission from ministers.

A government spokesperson said: “The PM and ministers are clear that the civil service does an outstanding job delivering for the public and driving progress on the government’s priorities.

“But when people and businesses across the country are facing rising costs, the public rightly expect their government to lead by example and run as efficiently as possible.”

Civil service unions are already at odds with ministers, led by Jacob Rees-Mogg, who are trying to pressure civil servants who have been working from home during the pandemic to return to Whitehall desks.

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Dave Penman, general secretary of the FDA union, tweeted: “Ultimately they can cut the civil service back to 2016 levels, but they need to decide what the civil service must then stop doing as a consequence.

“Will the Passport Office be cut back? Or the Dept of Health and Social Care?

“Unless they’ve got a serious plan, it’s either another headline-grabbing stunt or a reckless slash-and-burn to public services without a thought or care about the consequences.”

A Labour spokesperson said: “The cabinet said they would focus on the cost of living crisis facing families across the country.

“Instead of implementing an emergency budget they have chosen to let down working people once again through pointless rhetoric and lack of action.”