The prime minister wrote to civil servants in May justifying plans to cut the number of workers by almost 20%, saying the government needed to cut costs “as many families do”. But a review by Steve Barclay, Johnson’s former chief of staff, is said to have caused the Treasury to have second thoughts about the plans, given the potential impact on wider services. Johnson had instructed the cabinet to cut staff by a fifth, telling ministers during an away day in Stoke-on-Trent earlier this year that every bit of cash saved in government spending could be better used elsewhere. His plans are backed by Liz Truss, the star of the Conservative leadership, who has been accused of making “ridiculous” claims as she vowed to cut civil servant pay and cut spending to recover £11 billion a year in a “war against Whitehall waste’. But one Whitehall insider, who is said to have worked on the plans to ax tens of thousands of civil servants, was quoted in the Financial Times as saying the prime minister had announced the move without fully thinking through the implications. “You can only make 91,000 cuts with real cuts to major frontline services,” they added. “There is no way to get to that number through efficiency savings or cuts in head office staff.” Another Whitehall source was quoted as saying a figure of £2bn had been set as the cost of compulsory redundancy payments. Dave Penman, the general secretary of the FDA, a union representing senior civil servants, tweeted on Monday: “It’s almost like we’re cutting a fifth of the staff, based on an artificial number made up solely for political headlines that don’t takes into account the current demands on public services is a bad idea. “Ministers can determine the size of the public service, but they cannot hide from the consequences,” he added. Mark Servotka, general secretary of the Public and Commercial Services union, said: “We have long warned that cuts have consequences and that the original cut announcement was motivated by politics rather than practice. “If, as suggested, the Treasury is ‘cooling off’ on cuts, then we welcome the arrival of reality within government – you cannot ask more work from civil servants while employing fewer of them. “We will follow developments with interest and warn whoever is prime minister that we will fight for every job, against every cut. Our working members should receive an above-inflation pay rise to help them overcome the cost of living crisis, not a redundancy notice.” A government spokesman said: “As people across the country face huge living costs, the public rightly expect their government to lead by example and run as efficiently as possible. “Ministers have been tasked with drawing up plans to return the public service to 2016 levels over the next three years and work is ongoing.” Subscribe to First Edition, our free daily newsletter – every morning at 7am. BST The Guardian revealed in May that the Department for Environment, Food and Rural Affairs had already launched an effective hiring freeze and warned that redundancies could not be ruled out. The Prime Minister then wrote to civil servants to try to reassure officials that support would be provided to “anyone affected”. He argued that since Britain had left the EU and the Covid pandemic was subsiding, “we no longer require the state to have the same colossal presence in people’s lives”. The group behind Truss, who polls show is leading Rishi Sunak in the race to succeed Johnson, was forced to repeat some of its figures within hours of publishing a raft of proposals to cut the cost of the public service . These included less payments to staff outside London and a cut in annual holiday pay. Steven Littlewood, the FDA’s assistant secretary general, who represents senior civil servants, said: “It was clear from the start that these job cuts were based on an arbitrary number and were announced for headlines rather than a workforce strategy. “The fact that they are undeliverable without huge damage to frontline services will come as a surprise to anyone who doesn’t know how public services work.”