Rishi Sunak has refused to commit to wearing a mask inside a crowded House of Commons, as a leading government scientific adviser said ministers were mistaken to believe that vaccinations alone would keep Covid levels under control.

The chancellor also reiterated that ministers did not yet believe it was necessary to move to the government’s “plan B” for Covid over the winter, which would reintroduce mandatory mask wearing for crowded spaces, vaccine passports and more home working.

“At the moment, the data does not suggest that we should be immediately moving to plan B,” Sunak told BBC1’s Andrew Marr Show. “But of course we will keep an eye on that. The plans are ready, we outlined them way beforehand to make sure people knew what the possible options would be for the winter, which we said would be challenging.”

Sunak, who does not wear a mask in the Commons, dodged questions about whether this was the right thing to do in an often crowded, poorly ventilated chamber. He did wear a masks in other settings, such as crowded trains, he said.

“The government guidance is for people to make decisions based on what they think is appropriate based on the circumstances they are in,” he said. “Every workplace is going to be different depending on how many people are there, how long you’re there for, whether you know the people or not.”

This had been the consistent government line on mask use in the Commons, until the health secretary, Sajid Javid, said last week that Conservative MPs should set an example over mask use.

Adam Finn, a professor of paediatrics at Bristol University and a member of the government’s Joint Committee on Vaccinations and Immunisation, said relying on vaccinations alone, even with booster jabs, was not enough.

“I would like to re-emphasise the fact that the vaccine programme by itself, in the current situation, even if things go optimally, is not in my opinion enough to bring things under control,” he told Sky’s Trevor Phillips on Sunday show.

“We do need to have people using lateral flow tests, avoiding contact with large numbers of people in enclosed spaces, using masks, all of those things now need to happen if we’re going to stop this rise and get things under control soon enough to stop a real meltdown in the middle of the winter.”

Finn said he was concerned that the rate of vaccination was suffering because of a sense “that somehow the problem’s gone and we can all go back to normal again”.

“So I do think we need to see a very different kind of message coming from the government now that there is a serious problem, and we all need to contribute to reducing transmission, so that we can get through the winter and the NHS can stay afloat and absolutely we can avoid lockdowns, and the disasters that those bring.”

Speaking on the same show, Dr Katherine Henderson, the president of the Royal College of Emergency Medicine, said emergency departments were already in a “terrible place”.

“We’re already struggling to cope,” she said. “This is not something that’s coming in the next couple of months. We’re already in a terrible place where we have got large queues of ambulances with vulnerable people waiting in those ambulances to be offloaded into departments and other patients at home waiting to be picked up by the ambulance.”