A union boss has urged Boris Johnson to stop "micro-managing" civil servants, as the government pushes to end a working-from-home culture.
FDA general secretary Dave Penman accused the prime minister of "going to war" with those delivering his agenda.
Mr Johnson has backed demands for civil servants to return to the office after Covid-19 restrictions were lifted.
Cabinet minister Jacob Rees-Mogg has said all civil servants must stop working from home.
The minister has been criticised for leaving civil servants a note, which said "sorry you were out when I visited".
The note, printed on government paper with Mr Rees-Mogg's title, was left at empty desks and read "I look forward to seeing you in the office very soon".
At the FDA union's annual conference on Thursday, Mr Penman rebuked Mr Rees-Mogg for his "crass, condescending, passive-aggressive little notes for people who were actually delivering vital public services".
Jacob Rees-Mogg is leaving this note for civil servants who aren’t at their desks… pic.twitter.com/7KzBcGKVJP— Dino Sofos (@dinosofos) April 22, 2022
In a speech, Mr Penman said the FDA, which represents senior staff in the civil service, will "always be the pragmatic, constructive partner that employers should want to work with".
But, he added, it is hard to imagine a time in the FDA's 100-year history when the union has been needed more by civil servants.
In a message for the prime minister, Mr Penman said: "You say you want a brilliant civil service and you want to attract the brightest and best to join it.
"Well, this is not the way to go about it.
"Challenge us to deliver, be clear about your priorities, but step back and let those whose job it is to run the service get on with it.
"No more micro-managing, no more anonymous briefings. You do your job and let the management of the civil service get on with theirs."
'A quiet revolution'
The government's recommendation for people to work from home ended in January, after being re-introduced in the face of the emergence of the Omicron variant last year.
In April, civil servants were told they must stop working from home and return to the office to ensure government buildings are at full capacity.
At a cabinet meeting, the prime minister encouraged ministers and their departments to do "everything possible to speed up the return of more civil servants into the office", his official spokesperson said.
Mr Rees-Mogg wrote to cabinet colleagues urging them to send a "clear message" to the civil service about returning.
He also sent a league table of daily civil service office attendance from the week beginning on 4 April. It showed in some departments daily average of staff in the workplace was lower than a third.
But the FDA union has said Mr Rees-Mogg's approach to working arrangements were out of step with practice in the private sector.
In his speech, Mr Penman said private industry has embraced the "quiet revolution" in working practices over the past two years, delivering efficiencies for employers and greater flexibility for employees.
Meanwhile, Mr Rees-Mogg has been "wandering around Whitehall with his clip board and his clicker counting people at desks", Mr Penman said.
He added: "It's like he doesn't understand that the majority of civil servants are outside of the M25.
"Tens of thousands of civil servants in those famous red wall seats, all being told to forget about flexible working."