Ministers have ordered a local council in England to end its trial with a four-day week.
South Cambridgeshire District Council had announced plans to extend the experiment until next April.
But in a shock move the Government has ordered officials to end it now.
It was the first local authority in the UK to undertake the trial, while council bosses claimed it was showing positive results.
Local government minister Lee Rowley wrote to Liberal Democrat council leader Bridget Smith to 'ask that you end your experiment immediately'.
He says he had concerns about the “value for money” for local taxpayers.
It comes as there has been growing interest in four-day week experiments in the UK and worldwide, with some businesses in the private sector praising the shift to a shorter working week.
But Mr Rowley said the four-day week is inappropriate for local authorities.
In a letter written on Friday, he said: “As I am sure you are aware, all councils are expected to ensure that finite and valuable taxpayers’ money is used in a way which demonstrates value for money – something which paying employees for an extra day of work that is not carried out is unlikely to demonstrate,” he wrote.
“I strongly believe in the ability of councils to innovate and find new ways to discharge their responsibilities.
"Yet removing up to 20 per cent of the capacity to do those activities is not something which should be acceptable for a council seeking to demonstrate value for money for its taxpayers and residents.
“Whilst some private sector organisations may choose to experiment with their own capital and capacity regarding ‘four-day working weeks’, local government should not do the same.”
He has also warned that the move could breach the council’s legal duties under the Local Government Act.
The Department for Levelling Up, Housing & Communities will “shortly be issuing clear guidance” on the matter, Mr Rowley added.
“I look forward to your confirmation that South Cambridgeshire will be returning to established norms around local government workforce capacity in the coming weeks ahead,” he said.
Joe Ryle, director of the 4-Day Week campaign group, has blasted the news.
He said: “This move by the Government flies in the face of all the evidence, which shows the four-day week has been a huge success at the council.
“The four-day week with no loss of pay is already being being rolled out across the private sector so it’s only fair the public sector are included too.
“There is no good reason to end this trial, which is already bringing many benefits to council workers, local residents and saving the council money.”
The Liberal Democrat council leader Ms Smith recently told the BBC the move came after years of relying on agency staff to fill the gaps left by vacancies.
“On average we only fill about 80 per cent and we have some vacancies that we have been carrying for years, particularly in planning,” she told the broadcaster.
“We had to do something. We have already managed to deliver a third of a million pounds in savings, largely because we’ve now filled some of those really difficult to fill roles within planning.
“It means we’re delivering a much better planning service to our residents.”
