Civil servants have demonstrated a "Blitz spirit" by keeping keeping public services going while working from home, a union leader has argued.
Dave Penman, of the First Division Association, said they had "never worked so hard".
His remarks follow a Daily Mail report suggesting fewer civil servants have turned up at Whitehall since official guidelines changed at the weekend.
The government said it wanted to end a "default" towards working from home.
Until 1 August, the guidance for England was that every employee in the public and private sectors able to work from home should do so.
Since then the government has left it to employers to decide whether they need more people on the premises, safety permitting.
In July, the Home Office revealed that the passport application backlog was 400,000, blaming reduced staffing to allow for social distancing among staff.
The government has not provided specific return-to-office advice for civil servants separate to that for businesses and other organisations.
Mr Penman, whose union represents civil service managers, told the BBC that staff in Whitehall and elsewhere had been "performing incredibly effectively during this pandemic".
He added: "Designing and implementing the furlough scheme and processing a six-fold increase in Universal Credit claims - all while transforming into a home-based organisation overnight - is a perfect example of the Blitz spirit."
Mr Penman also said people were able to work "effectively" from home, adding: "Their job is to provide vital public services, not provide customers to sandwich shops."
Last month, the Financial Times reported that every Whitehall department - like private-sector employers - had been told to do its own Covid-19 risk assessment.
Most found they could return to 25% to 30% occupancy "without a threat to the health of staff", it added.
But Mark Serwotka, general secretary of the Public and Commercial Services Union, the UK's largest civil service union, accused ministers of "playing fast and loose with workers' safety".
He said: "No one should be returning to a workplace until it has been made safe to do so. Current scientific advice is that people should work from home where they can."
Mr Serwotka warned the change in return-to-work policy was "risking industrial unrest".
A government spokesman said: "Civil servants have worked throughout the pandemic and have made significant, valuable contributions towards the national effort.
"We are consulting closely with employees on ending the default that civil servants should work from home and have ensured workplaces are Covid-secure so civil servants can return safely."
Figures from the investment banking company Morgan Stanley suggest 34% of UK office workers are operating in their normal workplace.
This is less than half the proportion in France (83%), Italy (76%), Spain (73%) and Germany (70%), its survey of more than 4,000 people found.