A government minister has ordered a council to end its trial of the four-day week "immediately".
Local Government Minister Lee Rowley warned South Cambridgeshire District Council that giving staff an extra day off each week is unlikely to provide value for money to taxpayers - and may breach its legal duties.
South Cambridgeshire is the first council to introduce the four-day working week.
It began a three-month trial in January and then announced plans to extend it until next April.
Mr Rowley wrote to council leader Bridget Smith on Friday to “ask that you end your experiment immediately”, saying it was inappropriate for local authorities.
He said: “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 said such an approach could breach the council’s legal duties under the Local Government Act.
The Department for Levelling Up, Housing and Communities will “shortly be issuing clear guidance” on the matter, he added.
Council leader Ms Smith defended the scheme saying it had helped address the council's reliance on expensive agency staff, and reduced its annual bill by £300,000.
She said that performance was maintained and in some cases, improved in the first three months of the trial.
She said in a statement: “At the start of our trial we were carrying a £2 million annual agency bill. During the first three months of the trial, we filled four permanent posts that had previously been impossible to fill.
“This has reduced our annual bill by £300,000.
“As time goes on it is becoming increasingly clear that recruitment has been positively affected, both in terms of the quality and number of applicants, and the consequent success in filling vacant posts.”
She replied to Mr Rowley on Saturday, asking for a meeting with ministers.
A recent UK trial saw 61 companies reduce their working hours for all staff by 20% for six months. Most of them decided to maintain the shorter week.
Joe Ryle, director of the 4-Day Week campaign group, 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 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."
However, Elliot Keck, Head of Campaigns for The TaxPayer's Alliance, said: "We think it's a crazy plan, because ultimately such a radical experiment should be carried out by the private sector and not a small local council, which residents really rely on for services.
"And they don't want to see a part time council.
"The council claims that performance hasn't been impacted but actually on a couple of key metrics, for example, the time it takes to answer the phone or the time it takes to process housing benefit claims, both of those have seen a deterioration during the months of the trial."
The Department for Levelling Up, Housing and Communities “strongly believe in principles of local democracy and local councils can and should continuously look for ways to improve their services”, a spokesman said.
Asked about a potential meeting, the spokesman said: “Councils also need to ensure they are giving local taxpayers the best value for money and pressing ahead with the choice to remove up to a fifth of their capacity would need to be backed by clear evidence.”
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know