“Considerable” pressure on the NHS is likely to last for weeks to come, Boris Johnson has warned, as staff shortages threatened standards of care at the country’s hospitals.

The Prime Minister pledged to “make sure that we look after our NHS any way that we can”.

He warned “there’s no question Omicron continues to surge through the country” and he added: “I think we’ve got to recognise that the pressure on our NHS, on our hospitals, is going to be considerable in the course of the next couple of weeks, and maybe more.”

Mr Johnson said he appreciated the pressure NHS staff were under, and that it was “vital that we make sure that we help them by trying to contain the pandemic” by getting vaccinated and following plan B measures.

And he warned it would be “absolute folly to say that this thing is all over now bar the shouting”.

In a bid to stop the spread of the Omicron variant, the UK Government announced secondary school pupils in England would be asked to wear masks in the classroom again.

Mr Johnson said he was not happy about the idea but that it was a necessary step.

He said: “There’s an increasing body of scientific support for the idea that face masks can contain transmission.

Times Series: Image: PAImage: PA

“We don’t want to keep them. I don’t like the idea of having face masks in (the) classroom any more than anybody else does, but we won’t keep them on a day more than is necessary.”

Earlier, Education Secretary Nadhim Zahawi confirmed he had told headteachers to consider merging classes or sending groups of children home if the number of school staff off work due to Covid reaches critical levels.

He said it was key to keep children in the classroom as much as possible and to keep schools open due to the mental health impacts.

Caroline Derbyshire, executive head at Saffron Walden County High School in Essex, and leader of Saffron Academy Trust, told the PA news agency the practice of merging classes had already been carried out by schools “all term last term”, but it was “not a long-term solution”.

She said staff shortages would “absolutely” make remote learning more likely, adding: “If you hit a certain point with staff absences in a big school you’re talking about maybe 10 members of staff being off.”

But Mr Zahawi told Sky News: “The priority is to keep schools open.”

Mr Zahawi said: “The testing, the staffing support we’re putting in place, and of course the ventilation is going to make a big difference to schools this year.

“The most important thing is to keep them open.

“We monitor staff absenteeism, I just said to you we’re running at about 8% last year.

“If that rises further then we look at things like merging classes, teaching in bigger numbers.”

Mr Zahawi admitted it was “more challenging, of course, to deliver education with masks on in the classroom”.

But he said: “This is an aerosol-transmitted virus and if you’re wearing a mask, if you’re asymptomatic, then you’re less likely to infect other people.”

He also said the data did not currently support the extension of coronavirus measures when plan B guidance is up for review on Wednesday, while Mr Johnson suggested he would stick with the current advice.

Mr Johnson was asked whether resisting further restrictions would risk putting further strain on the NHS.

He said all measures would be kept under review but he said: “I think the way forward for the country as a whole is to continue with the path that we’re on.

“We’ll keep everything under review.

“The mixture of things that we’re doing at the moment is, I think, the right one.

“So, number one, continue with Plan B, make sure that people take it seriously, do what we can to stop the spread, use the Plan B measures, work from home if you can, wear a mask on public transport… take a test before going out to meet people you don’t normally meet, think about the requirements under Plan B, but also get the boost.”