Support staff at the Department for Business Energy and Industrial Strategy have voted to strike over concerns about coronavirus safety measures.
A ballot of 95 cleaners, porters, security staff and postage-service workers showed staff were overwhelmingly in favour of a walkout and provided a mandate for action, the PCS union said.
PCS said ISS, the contractor that employs the support staff, had failed to wind down support services sufficiently to enable members to stay at home.
Although many civil servants are working from home, the union said BEIS’s role in preparing for the end of the Brexit transition period meant some officials were having to come into the office.
As a result, support staff are forced to come into the department’s Victoria Street headquarters to keep the offices running, it said.
PCS said four ISS employees had tested positive for coronavirus.
The vote comes a month after three unions wrote to BEIS permanent secretary Sarah Munby raising "extreme concern" about social distancing at the department's offices.
PCS has once again called on BEIS to reduce the number of civil servants working from the Whitehall HQ to prevent contractors “unnecessarily endangering their health”.
In the ballot, 94.3% voted for strike action on a 73.7% turnout. PCS noted that the overwhelming majority of support staff are from black and minority ethnic backgrounds communities, which have been disproportionately affected by the Covid-19 pandemic.
PCS general secretary Mark Serwotka said: "BEIS and ISS are continuing to put our members at unnecessary risk by not allowing support staff to stay at home.
“We hope this huge mandate for action will make both ISS and BEIS management take heed of our members’ serious concerns.
"Many civil servants are working well from home and there should be no need for our support staff to continue to put themselves in harm's way at BEIS.”
An ISS spokesperson said: "ISS is disappointed that some PCS union members have voted to take industrial action. ISS remain vehemently committed to the wellbeing of every employee and continue to follow all Covid-19 guidelines set by Public Health England and the government to ensure that all of our people are protected and safe.
"We are fully compliant with health and safety regulations and follow the required process and procedures at all times. We continue to ensure our employees, customers and the workplaces in which we work continue to operate in the safest way possible."
A BEIS spokesperson said: “While the vast majority of our staff continue to work from home, a small number of staff who cannot carry out their normal duties from home are continuing to use the building.
“We have taken extensive measures to make the building Covid-secure and safe, and we continue to support working arrangements to reduce the transmission risk and provide extra support where needed.”
Measures include staggering start and end times for shifts, and agreeing with ISS that staff who are unable to work in the office because they are shielding or have caring responsibilities, can stay at home on full pay.