Education secretary Gavin Williamson has been warned he could face legal action from teachers after fresh Covid test-and-trace failures left schools unable to check if pupils have got the virus.

The teaching union NASUWT has written to Williamson to state it could sue over a breach of a duty of care and personal injury to staff caused by the reopening of schools without proper safeguards.

And in a separate letter to schools minister Nick Gibb, the union’s general secretary Patrick Roach has revealed that a dedicated testing centre for schools in Salford has had to turn away requests because of a surge in demand.

The letter, seen by HuffPost UK, states that “the number of symptomatic pupils and staff has increased to such levels that the testing site has been unable to cope with demand and has stopped taking referrals from schools”.

The union also said that in Bury, Greater Manchester, some 600 pupils are now self-isolating but testing was being overwhelmed.

Roach said that the reopening “risk assessments” that all schools were advised to carry out by the government depended on a functioning test and trace system that was currently lacking.

One source with knowledge of the test and trace system in Greater Manchester said that the delays in test turnarounds risked undermining the entire system of year group “bubbles” of up to 300 children, where pupils are expected to self-isolate if one of their number tests positive. 

“Under the current guidance, a pupil could be sent home with symptoms today, not be able to get a test until Friday and then wait another few days for the results,” they said.

“Bubbles and contacts are not being isolated until the positive result is confirmed, which means that potentially, many unidentified confirmed cases remain in circulation, increasing the risks to teachers and their pupils, and, of course, the wider community.

“Even if a school follows the guidance and send symptomatic pupils and staff home, they cannot implement the next stage of their risk assessment without swift testing.”

In his letter, Roach said the union had “numerous examples” of problems with testing and schools operating blind when pupils were sent home with symptoms.

He told Williamson that the union was “expressly reserving our members’ legal rights” in the case of a claim for breach of duty of care or personal injury due to foreseeable risks from reopening schools.

Some schools have closed their doors days after reopening this term, while others have told year groups to self-isolate for two weeks following confirmed cases.

Figures from the Department for Education (DfE) showed that around 92% of state schools were fully open on Thursday last week, and approximately 88% of students were back in class on the same day.

But the NASUWT said the DfE had been “unable to provide any evidence on the effectiveness of the risk control measures recommended in your guidance to schools.”

The union also released an online snapshot poll of its 900 members conducted over the past two weeks, suggesting that nearly a third (31%) do not have access to soap and water for themselves and their pupils.

It found that just 18% had hand sanitiser in every classroom and only a quarter had one-way systems or staggered start times for pupils.

The majority (55%) of teachers said they did not believe the Covid-19 safety measures introduced by their school were “sufficient and effective”.

The letter adds: “We also seek confirmation from you that you have obtained assurances that the implementation by schools of your decisions on the reopening of schools are not in breach of schools’ legal obligations relating to health and safety, employment or equalities.

“Therefore, the NASUWT is putting the Government on notice by expressly reserving our members’ legal rights in the context of a tortious claim for breach of duty of care and personal injury due to foreseeable risk, and any other legal recourse available.”

In the letter, Dr Roach insisted: “For the avoidance of doubt, the NASUWT is and remains committed to ensuring that schools remain open safely.”

A DfE spokesperson said: “Schools have implemented a range of protective measures, based on the Public Health England endorsed ‘system of controls’, which create an inherently safer system to minimise the risks of transmission.

“This includes reducing mixing and distancing where possible, including by staggering break and lunch times, as well as increasing the frequency of cleaning and handwashing.

“Figures show that on September 10 99.9% of state-funded schools were open to pupils, and we will continue to work closely with schools to ensure all appropriate steps are taken to keep pupils and staff safe.”