The government’s back-to-school plans are in chaos after leadership unions began legal action and the country’s largest teaching union told its members they don’t have to work in unsafe environments.

The government announced on Wednesday that most primary schools will reopen next week, while the start of term for secondaries was pushed back to January 11 for exam pupils, and January 18 for all others.

Primaries in Covid hotspot areas will remain closed for two weeks. The additional measures are meant to help combat rising case numbers and the spread of a new variant of Covid-19.

However the government has already U-turned on two aspects of the plan. Yesterday it announced all mainstream primary schools across London would close after some boroughs told to open their schools challenged the decision. Mass testing in secondary schools has also now been made mandatory.

But pressure on ministers’ plans has today been ramped up after the NAHT leadership union said it had taken preliminary steps in legal proceedings against the Department for Education.

According to the union, the process covers “a wide range of issues from the scientific advice the government is drawing on, right through to the proposed arrangements for covid testing in schools”. The ASCL union has also joined the action, and the DfE has until 4pm on Monday to respond.

“This represents a significant step for us to have taken as a union, and it is not a decision we have taken lightly,” said NAHT general secretary Paul Whiteman in an email update to members.

“Once we have seen the response, we will need to rely on our appointed Queen’s Counsel and the NAHT and ASCL legal teams to determine the likelihood of success should we continue with the legal action. If at that point we need to take a different approach, we will be prepared to do so.”

ASCL also convened an emergency meeting of its executive committee today to discuss the union’s position. At that meeting, the union took the decision to also call for face-to-face teaching to be provided “only to vulnerable children and children of key workers until at least January 18”.

“We are calling for a short period of remote education in order to protect all concerned and allow time for the government to work with the profession on a joint plan for safe opening,” said general secretary Geoff Barton.

“We fully support keeping education functioning as fully as possible during the Covid crisis but this has to be done safely, or the long-term consequences and disruption will be much worse.”

It follows calls from the National Education Union, England’s largest education union, for a move to learning online in all primary schools in England for at least two weeks.

The NEU has also issued advice to its members reminding them of their legal rights not to have to work in an unsafe environment.

Kevin Courtney, the union’s joint general secretary, said it was “not good enough to always be behind the curve, playing catch up with new strains of Covid, seeing hospital admissions rise and cases numbers spiral out of control”.

“Whilst we are calling on the Government to take the right steps, as a responsible union we cannot simply agree that the government’s wrong steps should be implemented.

“That is why we are doing our job as a union by informing our members that they have a legal right to refuse to work in unsafe conditions which are a danger to their health and to the health of their school communities and more generally.”

Section 44 of the employment rights act 1996 states that employees have the right “not to be subjected to any detriment” if they leave or refuse to return to work because they believe it is dangerous.

Unison, which represents around 250,000 school support staff, is advising its members that most staff don’t need to go in next week.

“We will be joining our sister unions in recommending that only staff needed to look after vulnerable and children of key workers should return to schools on Monday, similar to the first lockdown,” said Unison’s head of education Jon Richards.

It comes after minutes published by the government’s Scientific Advisory Group for Emergencies (Sage) show the group advised ministers on December 22 that it was “highly unlikely that measures with stringency and adherence in line with the measures in England in November [with schools open] would be sufficient to maintain R below 1 in the presence of the new variant”.

NASUWT, the country’s second largest teaching union, has also called for a nationwide move to remote education for all schools.

A DfE spokesperson said: “Children’s education has consistently been a national priority, which is why we want classrooms to reopen wherever possible in the new term. Schools will continue to implement appropriate safety measures to help mitigate the risk of transmission.

“As we’ve said, we will move to remote education as a last resort, with involvement of public health officials, in areas where infection and pressures on the NHS are highest.”