The government's welfare department has been rapped by an official watchdog for breaching health and safety law.
The Department for Work and Pensions had 'insufficient' social distancing at its Leeds office, an investigation found.
Photos showed staff congregating round a computer screen - and a thin walkway that passed close to desks where people could work.
Leaked messages obtained by the BBC also claim there had been at least two positive Covid-19 cases among staff.
The Health and Safety Executive found "contraventions of health and safety law" after a visit on August 25 and demanded urgent changes.
The HSE told DWP chiefs: "You are failing to ensure, so far as is reasonably practicable, the health and safety of your employees at work because you have not implemented necessary measures to prevent the spread of COVID-19."
Managers were aiming to have 50% of staff back in the Quarry House office - but the HSE said this "may be ambitious and unrealistic".
In a scathing report, the HSE said "you (DWP) have not, where possible maintained social distancing."
The watchdog said there "doesn’t appear to be any consideration or assessment" of the potential for crowding at entrance and exit doors.
There were no "do not use" signs on tightly-packed breakout pods and areas where staff might sit.
And many walkways "are little more than 1m wide and pass very close to desks that are designated as useable".
The findings were slammed by the PCS union, which represents many DWP staff.
General secretary Mark Serwotka said the government has "failed staff and the wider community by not adhering to basic health and safety legislation.
"There is a very real danger of a second Covid spike.
"This incident makes a mockery of DWP's insistence that offices open until 8pm to the public when they can't even maintain social distancing.
"It is unlikely this is an isolated incident and Boris Johnson should re-think forcing thousands of civil servants back to the office when the rate of covid infection in the country is rising significantly."
A DWP spokesman said: “We take the health and safety of staff extremely seriously, and have implemented COVID-secure measures across our sites to ensure they comply with government guidelines.
"We have taken urgent action to rectify all issues identified by the HSE.”
