Local government minister Lee Rowley wrote to South Cambridgeshire District Council to ask for an immediate end to the trial.
A local Cambridgeshire council is seeking a meeting with ministers after being ordered to end its experiment with a four-day week.
South Cambridgeshire District Council had announced plans to extend its trial until next April but ministers have ordered officials to end it now.
It was first local authority in the UK to undertake such a trial.
Local government minister Lee Rowley wrote to Liberal Democrat council leader Bridget Smith to “ask that you end your experiment immediately” and say he had concerns about the “value for money” for local taxpayers.
Ms Smith, in a reply to the minister on Saturday, said she was “surprised” to receive the letter and asked for “a meeting with ministers to discuss this matter”.
There has been growing interest in four-day week experiments in the UK and globally, with some businesses praising the shift to a shorter working week.
But Mr Rowley, in a letter dated June 30, said such a model is inappropriate for local authorities.
“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% 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 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, 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.
In her letter, Ms Smith defended the scheme and said staff continue “to improve the council’s ability to provide high quality, consistent, resilient services, and save taxpayer money”.
In a statement, the council leader said: “I was surprised to receive Mr Rowley’s letter and we have written to him to request a meeting with ministers to discuss this matter.
“This is a trial, but we have already seen strong independently assessed evidence which showed that performance was maintained, and in some cases improved, in the first three months.
“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.”
Campaigners for a four-day week also criticised the move.
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.”