A public sector union said it would be willing to consider strike action after confirming it opposed Cabinet-driven plans for the vast majority of civil servants to return to the office by the end of the month.

Outgoing Cabinet Secretary Sir Mark Sedwill, in a letter seen by the PA news agency, has written to the permanent secretaries of Government departments calling on them to bring as much as 80% of public sector staff back into the workplace.

Sir Mark, in the note dated September 3, said the Prime Minister had asked to be personally involved in the back-to-work drive and wanted to see departmental figures on a “weekly basis” following Cabinet agreement that increasing office numbers would be “hugely beneficial for our workforce”.

But the instructions have faced backlash from unions representing the Civil Service, with the Public and Commercial Services (PCS) union confirming it would, as a “last resort”, consult members about possible industrial action.

In a statement, the union said: “Our members have kept the country running during the pandemic while working from home, and we believe it is not safe to return to workplaces while Covid-19 infection rates remain high and given the likelihood of a second wave in the coming weeks.”

PCS general secretary Mark Serwotka said: “If the Government or any employer starts forcing people back to work and we believe that it is not safe to do so, we will firstly consider our legal options, secondly give individual legal advice, and thirdly consider whether a collective response is required.

“As a last resort, if you have no other option and people’s health and safety is at risk, of course we would be prepared to consider industrial action.”

The PCS national executive committee is due to meet on Wednesday September 9 to “decide how to respond”, the union said, with officials in the meantime asking departments to provide the Covid-secure limit and current staffing levels in each building where its members work.

The Prospect union said the setting of “arbitrary timescales and targets is wrong” and called for a “managed and graduated approach” to continue.

A third union, the FDA, meanwhile, labelled the bid to have the public sector lead the return to pre-coronavirus work patterns as “virtue signalling” to a private sector that had adapted well to remote working.

General secretary Dave Penman told BBC Radio 4’s Today programme the Civil Service had worked “effectively” since the pandemic started, dealing with the increased demand for Universal Credit and starting the furlough programme from the ground up, all with staff being 95% home-based.

Downing Street this week denied the existence of a “back-to-work” campaign, but Sir Mark’s letter is yet further evidence that ministers fear huge job losses in town and city centre shops and cafes if workers do not return to their pre-lockdown commuter patterns soon.

Boris Johnson is keen for politicians and the Civil Service to provide the example for the rest of the country, reportedly telling Tory MPs he wants the Commons “back to normal” by Christmas.

Sir Mark said departments which were below their coronavirus staffing constraints “should now move quickly to seek to bring more staff back into the office in a Covid-secure way”.

He wrote: “We are now strongly encouraging an increased workplace attendance through staff rota systems, with our aim by the end of September to enable 80% of staff to attend their usual workplace each week, for example 20% for five days, 30% for three days and 30% for two days, with the balance attending only occasionally for now.”

Figures published on Friday suggest employees are beginning to curtail their work from home habits.

According to the latest data from the Office for National Statistics (ONS), there has been an increase in people travelling to work in the last two months, with fewer working exclusively from home.

Transport for London also reported an increase in passenger numbers throughout the tube and bus network on Thursday and Friday.