Fewer school staff may be “willing or able” to work following the prime minister’s move to lock down Britain in the absence of clear advice on how to stay safe in schools, headteachers have warned.

Many staff will be “rightly” concerned about their families and anxious about the risks of going to work after hearing Boris Johnson’s announcement, according to the NAHT school leaders’ union.

Schools in England officially closed on Friday but have remained open for vulnerable youngsters and the children of key workers, such as NHS staff, police and delivery drivers.

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In a message to members, NAHT general secretary Paul Whiteman said: “We know that the vast majority of schools opened yesterday offering reduced provision. Our survey showed that most schools operated with fewer than one in four teaching and support staff present.

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“Having heard the prime minister speak, many colleagues will be rightly concerned for loved ones and understandably anxious about the personal risk of simply going to work. It would not be surprising to find, in the absence of clear government advice on keeping safe, that fewer colleagues are willing or able to work today than yesterday.”

Mr Whiteman added that in recent days, school leaders and their teams had shown “almost infinite reserves of resourcefulness” as he called for a “step-change” in advice given to schools.

“Our work with the Department for Education is productive but we need a step-change in the detail of advice given to schools now,” he said.

“Our focus will be to reiterate the need for schools to be given clear guidance on how to keep pupils and staff safe.

“It is absolutely vital that we have sight of the expert medical evidence on safe levels of attendance and density, and are provided with practical advice on the protective steps we should be taking in schools.

“From masks or other PPE [personal protective equipment], to distancing children from each other, to sufficient supplies of soap and hand sanitiser, schools urgently need answers to their questions about effective safety measures.”

There also needs to be more guidance for special schools and alternative provision, as these are taking in higher percentages of pupils, he said.

“School staff know that they are taking a risk by reporting to  work,against advice given to the rest of the population to stay at home,” Mr Whiteman added.

“It is only right that this is a choice informed by hard evidence regarding the degree of risk involved.”

Education secretary Gavin Williamson has thanked school staff for their efforts.

In a tweet, he said: “Thank you to all the teachers and staff working hard to support those who need it most.

“Every one of us is enlisted in the fight against this virus.”

Early indications suggest that the numbers of pupils arriving at schools is lower than first expected by school leaders, as parents heed advice to keep their children at home unless they have no other option.

An NAHT survey of 3,350 members has found that 94 per cent of schools are providing emergency cover for pupils, and 84 per cent of them have less than a fifth (20 per cent) of their usual children attending.

A separate snapshot survey of 670 schools, mainly secondary, showed that in most cases, fewer than 10 per cent of pupils were in school on Monday.

The Association of School and College Leaders, which conducted the latter poll, said that in 86 per cent of schools, the percentage of pupils attending lessons was between 1 per cent and 5 per cent.

Most of the schools were in England (94 per cent), with the rest in Wales and Northern Ireland.