All state schools in London could be affected by walkouts in the autumn term if co-ordinated strike action by teachers and headteachers goes ahead.

Four education unions, which represent the majority of school leaders and teachers across England, have said they will join up for any future industrial action in a long-running dispute over pay.

Kevin Courtney, joint general secretary of the National Education Union (NEU), said up to 400,000 teachers in England could be involved in walkouts in the autumn if all the unions go out on strike.

Currently only the NEU has a mandate to take strike action, with the next walkout planned for Tuesday, and it plans to re-ballot its teacher members in England to take further action in the autumn.

The National Association of Head Teachers (NAHT) and the NASUWT teaching union - which both failed to meet the mandatory 50% turnout threshold required for strikes in England in their last ballots - will re-ballot members in England during the summer term.

The Association of School and College Leaders (ASCL) is also due to hold a formal ballot for national strikes in England for the first time in its history.

Asked about the impact of possible co-ordinated strike action at a press conference at the NAHT’s annual conference in Telford, Mr Courtney said: “I think with our four unions you would find that every state school in England would be affected by the dispute and that would put you up at 300,000-400,000 teachers... involved in taking the action, I would have thought.

“We don’t want to take it. We want to find a solution. But with all four of us acting together I think we will all pass the Government’s undemocratic thresholds and so it would be an enormous response from our members.

“We would sincerely apologise to parents for disrupting their children’s education if we’re pushed to that. And we would sincerely apologise to them for disrupting their home and their working lives. However, what we are seeing is disruption in children’s education every week of the school year.”

Paul Whiteman, general secretary of the NAHT, told the press conference: “I have been around a decade and I have never seen the co-ordination that we are seeing here.”

Asked if the education sector could face the biggest strikes on record if all ballots are successful, Mr Whiteman said: “The steps now are to get through the thresholds and then to sit and meet and discuss what co-ordinated action actually looks like once we get through those thresholds.

“So it is a difficult question to answer one way or the other. Potentially yes, but we don’t know what that action looks like right now.”

Delegates at the NAHT’s annual conference in Telford on Friday afternoon unanimously passed a motion to ballot for strike action in the dispute over pay.

The Government’s recent pay offer was described as “derisory” and an “insult” by delegates at the conference.

After intensive talks with the education unions, the Government offered teachers a £1,000 one-off payment for the current school year (2022/23) and an average 4.5% rise for staff next year.

But all four education unions rejected the offer.

The decision on teachers’ pay in England for next year has been passed to the independent School Teachers’ Review Body (STRB).

On Friday, the NEU wrote to Education Secretary Gillian Keegan to give her formal notice that the union would be balloting its teacher members in England from May 15 to July 28 for further strike action.

Mary Bousted, joint general secretary of the NEU, said: “The Secretary of State who remains, by some distance, the biggest obstacle to getting a sensible resolution, needs to address this issue head on and come to the negotiating table with all the education unions. This wilful lack of engagement will be something that parents and teachers will not forget.”

A Department for Education spokesperson said: “For unions to co-ordinate strike action with the aim of causing maximum disruption to schools is unreasonable and disproportionate, especially given the impact the pandemic has already had on their learning.

“Children’s education has always been our absolute priority and they should be in classrooms where they belong.

“We have made a fair and reasonable teacher pay offer to the unions, which recognises teachers’ hard work and commitment as well as delivering an additional £2 billion in funding for schools, which they asked for.”