Council leaders have been warned not to roll out four-day working weeks by Local Government Secretary Steve Reed.
The letter is thought to have said that "council staff undertaking part-time work for full-time pay without compelling justification would be considered an indicator, among a wide range of factors, of potential failure", according to the Telegraph.
In a sharp sign off, he said he hoped he had made the government's position on the matter "unambiguously clear to all councils".
According to a Labour source: "Voters deserve high standards and hard work from local councils, and seeing council staff working a four-day week just won't cut it.
"They should get on with the job and make sure residents get the best service possible five days a week."
In the letter, the Telegraph reported that Reed believed councils should not be offering "full-time pay for part-time work".
Previously, Reed emphasised his “deep disappointment” after the first council in the UK adopted a four-day working week.
The Liberal Democrat run South Cambridgeshire District Council made the shift after testing out the new system in 2023.
Reed defended his position by saying that there had been a decline in the performance of the local authority housing service and questioned how the council would "mitigate" this.
The government is able to intervene if councils are failing.
This comes a year after Prime Minister Sir Keir Starmer rejected civil servants' calls for a four-day working week.
It has been argued that four-day working weeks can slow productivity and economic growth.
However, a trial of the scheme in the Scottish public sector saw that it boosted productivity and staff well-being.
Of those surveyed, 98 per cent of staff said morale and motivation had improved, according to the Autonomy Institute, who co-ordinated the Scottish government commissioned pilot.
