Tens of thousands more teachers have joined the UK’s largest education union to take part in strikes on Wednesday, a union boss has said.

Kevin Courtney, joint general secretary of the National Education Union (NEU), said ministers should be concerned about the 40,000 new sign-ups to the union since the teacher strikes were announced a fortnight ago.

He said the new members, of which the vast majority are teachers, are joining the union “because they want to be part of the action”.

Mr Courtney told the PA news agency: “That’s a very big conscious decision to make, to join us at this moment. If I was the Government, I’d be worried about that.”

The UK is facing the biggest day of industrial action in more than a decade on Wednesday as teachers, university staff, train drivers, civil servants, bus drivers and security guards will all go on strike.

Downing Street has conceded that Wednesday’s mass strike action will be “very difficult” for the public.

Walkouts by teacher members of the NEU in England and Wales, which threaten disruption to 23,000 schools, are the first of seven days of strikes by the union in February and March amid a pay dispute.

The NEU is estimating that around 85% of schools in England and Wales will be fully or partially closed on Wednesday.

Some parents will be forced to work from home and take leave to look after younger children due to school closures.

Jonathan Broadbery, director of policy at the National Day Nurseries Association (NDNA), has warned that the strike could have “quite a serious knock-on effect” for nurseries as staff may struggle to find childcare.

But Mr Courtney said parents have been largely supportive of the strikes – with some expected to join demonstrations on Wednesday – as they can see “schools have been run down” and have teacher shortages.

He told PA: “It’s a strike against disruption in education. We want the strike to be effective because we want to concentrate the Government’s mind on solving the issue.”

Education Secretary Gillian Keegan met the general secretaries of unions representing teachers and headteachers on Monday, but the talks failed to find a resolution.

Some 300,000 teachers and support staff were asked to vote in the NEU ballot – and more than 127,000 teacher members in England and Wales backed strike action.

In addition, new sign-ups to the NEU have nearly doubled in a week, from 22,000 on Tuesday last week to 40,000.

Mr Courtney told PA: “I think it will be very likely that all the yes voters and all the new joiners will take action, but that more will take action. Some of the no voters and some of the non-voters will take action as well.”

The NASUWT teachers’ union failed to achieve the 50% ballot turnout required by law for its members to go out on strike over pay in England, but it is advising members not to cover the work of NEU members who are striking.

In a message to parents on Wednesday, Mr Courtney said: “The strike will disrupt their child’s education and we regret that, and it will disrupt their home life and their work life and we regret that.

“We sincerely regret it, but we’re pointing to disruption that is happening every day in schools. If we don’t persuade Government to invest in education that disruption just carries on.”

The NEU is required by law to provide schools with the number of members that it is calling on to take strike action in each workplace, but the union does not have to provide the names of members.

School leaders will not necessarily know who is going to be available for work until the day itself

Headteachers and academy trust bosses are unlikely to know about any extra staff who have joined the NEU in order to strike since the results were announced just over two weeks ago.

Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said: “School leaders will not necessarily know who is going to be available for work until the day itself.”

He said heads “are prioritising in their planning students who are taking vocational exams scheduled that day and mock exams in preparation for GCSEs and A-levels, as well as vulnerable students, while for other students there is likely to be a mixture of on-site provision and learning from home”.

Seamus Murphy, the CEO of Turner Schools, which runs five schools in Folkestone, said three of the schools will not have all year groups in on Wednesday due to the strike action.

Pupils in years 3, 4 and 5 at one of the primary schools will be at home unless they are children of key workers or vulnerable, and in the two secondary schools years 8, 9 and 10 will have online learning.

Mr Murphy told PA: “There’s a real challenge here for the Government and I recognise the climate they’re in and the economic circumstances, and obviously from a personal point of view I am concerned about the level of disruption to children’s learning.

“But actually our longer term challenge is around recruiting staff who want to join and stay in a profession which is challenging.”

Thousands of ambulance workers across five services in England will strike on February 10 in the long-running dispute over pay and staffing, Unison announced on Tuesday.

Strikes will now be happening across the NHS every day next week apart from Wednesday.

NHS consultants in England are also gearing up for possible strike action.

The British Medical Association (BMA) – the country’s biggest doctors’ union – is to hold an indicative ballot of its consultant members in February in a dispute over pay and pensions.

The move comes after members of the Fire Brigades Union (FBU) voted overwhelmingly in favour of walk-outs in a ballot result announced on Monday.