Treasury documents have suggested that a return to home working, a key plank of Boris Johnson’s “plan B” proposal to deal with rising Covid-19 cases, would cause up to GBP18bn of damage to the UK economy over five months.

A government source said there was no suggestion restrictions would be that length, if they were introduced at all. Johnson has so far resisted a move to plan B in England, which would also entail more widespread mask-wearing and the extended use of vaccine passports. Instead, the government has said it will focus on ramping up booster jabs for the over-50s and vulnerable adults, as well as the vaccine programme for over-12s.

The documents, leaked to Politico, were drawn up by the Treasury and the Cabinet Office’s Covid-19 taskforce on the basis that a move to plan B would last until March 2022.

They also suggest that a more widespread use of Covid certification for mass events would reduce transmission at those venues, but suggested it would have only a moderate impact overall, because a relatively low amount of transmission takes place at those larger venues.

A government spokesperson declined to comment on the leak and said that timeline for plan B did not reflect current thinking. “We knew the coming months would be challenging, which is why we set out our autumn and winter plan last month. Plan B ensures we are ready, should we need to act, to avoid an unsustainable rise in hospitalisations that would put unsustainable pressure on the NHS.

“The presumptions put forward do not reflect government policy. The data does not currently show that plan B is necessary – and there is no planned five-month timeline.”

Government sources have confirmed that any move to plan B would involve masks, home working and Covid passes.

The leak of the documents, which suggest that a move to plan B would have high economic cost and only a moderate effect on transmission, are likely to be a useful tool as the government comes under pressure over rising Covid cases.

Ministers have been particularly criticised for not wearing masks in the House of Commons chamber, which is set to be packed on budget day on Wednesday. The health secretary, Sajid Javid, said he would wear one, but that it was a matter of personal choice.

On Tuesday, Dr David Nabarro, the WHO’s special envoy for Covid-19, said that “everybody” should be wearing masks in close confinement with other people, “including our leaders”.

“This virus, it is absolutely unstoppable, it gets everywhere, and so we have to do everything we possibly can to stop it,” he told Sky News.

“And one of the best ways to stop it is a well-fitting surgical mask properly over your face, pushed in over your nose, covering everything, and that reduces the risk to others and the risk to you. If it works, why on earth don’t people use it? It’s not a party-political issue – this virus doesn’t vote.

“And indeed, there’s no difference in how you deal with the virus when you vote for this party or that party. So everybody, wear masks when you are in close confinement, it’s the right, sensible, proper thing to do, and everybody should be doing it, including our leaders.”