Toughened coronavirus measures in schools will not be in place “for a day longer than we need it”, the Education Secretary has promised, as staff shortages were feared across the sector.

Nadhim Zahawi has told headteachers to consider merging classes or sending groups of children home if the number of staff off work due to Covid reaches critical levels.

But 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.

“The priority is to keep schools open,” he told Sky News, following the announcement that masks would be recommended in the classroom for secondary school pupils, as well as twice-weekly testing.

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.”

And he said he hoped guidance that secondary school children should wear masks in the classroom again would not be in place “for a day longer than we need it”.

It comes as the Prime Minister has ordered ministers to draw up contingency plans to tackle staff shortages caused by coronavirus across industries.

There are particular concerns about the NHS, with The Sunday Times reporting that United Lincolnshire Hospitals NHS Trust had declared a “critical incident” with “extreme and unprecedented” staff shortages resulting in “compromised care”.

The trust’s medical director Dr Colin Farquharson said: “As a result of significant staffing pressures due to absence related to Covid-19, we are having to take additional steps to maintain services.

“Our staff continue to work exceptionally hard and we would like to reassure our patients and the public that in spite of the challenges faced, essential services remain fully open for anyone who needs them, so people should continue to come forward for care.”

In Yorkshire, the region’s ambulance service said: “The added challenge of Covid-19-related absence amongst staff… is having a significant impact on our frontline operations.”

Chris Hopson, chief executive of NHS Providers, which represents health trusts, told the BBC: “We’re seeing increasing staff absences, and that’s coming on top of a very significant amount of wider pressure.”

There’s nothing in the data that gives me any concern that we need to go beyond where we are at

But Mr Zahawi said the NHS was “very good at being able to move staff around”.

The Education Secretary said the NHS was used to dealing with staff being off over the winter and during “big flu viruses”.

He told BBC Breakfast: “The NHS is very good at being able to move staff around within the system. They have an infrastructure to do that. We now have 10,000 more nurses and 3,000 more doctors than we had last year working in the NHS.

“But the NHS is very good at sort of making sure that staff shortages are monitored and dealt with pretty well. They’ve done it over many years in winter when we’ve been, you know, have big flu viruses around.”

It was put to Mr Zahawi that the health service had never had to deal with a situation like coronavirus and he said: “I don’t disagree with you.”

Despite the concerns over staff absences, Mr Zahawi said “there’s nothing in the data” to suggest further coronavirus measures will be needed later this week.

Mr Zahawi told BBC Breakfast that Plan B measures would be reviewed on Wednesday, but added: “There’s nothing in the data that gives me any concern that we need to go beyond where we are at.

“There’s some really good data from London that it looks like the infection rates are plateauing, if not yet coming down.

“But we are seeing leakage into the over-50s in terms of infections, and it’s generally the over-50s who end up with severe infection and hospitalisation.”

In a letter to Health Secretary Sajid Javid, the Royal College of Nursing said the differences in restrictions across the UK were causing confusion.

The RCN’s council chair Carol Popplestone and general secretary Pat Cullen said: “We continue to navigate the biggest health and care challenge the world has seen.

“It is confusing and concerning that the different UK governments have set out their own different rules and regulations in relation to the management of the pandemic.

“Nursing professionals are questioning the level and nature of the variation between governments.

“We therefore ask that, as Secretary of State for Health and Social Care, you work with counterparts across government on a more cautious approach for England without further delay.”