The Labour leader Ed Miliband joined David Cameron in urging trade unions not to press ahead with tomorrow's mass strike, as the scale of the disruption from the walk-out became clear.

In addition to the 2 million children likely to be given the day off as their teachers man picket lines at the school gates, air passengers were advised not to travel because of striking immigration staff. The courts system has plans to run a skeleton service.

Those on the picket lines are expected to include 999 call handlers, security officers at the Houses of Parliament and police support staff. Driving tests could be cancelled and job centres forced to close.

Up to 750,000 teachers, lecturers and civil servants may join the 24-hour walkout over the Government's proposals to make their pensions less generous. Protests are expected throughout Whitehall. Ministers, who face having to cross picket lines to get into their departments, made further threats to rewrite the strike laws yesterday. The National Union of Teachers predicted that 85 per cent of schools would be either shut or partially closed.

By last night, 3,206 schools had told local authorities they would be closed while 2,206 would be partially closed. Of the academies, 84 would be closed, 128 partially closed and 158 fully open.

However, government sources admit that they do not know the overall impact – partly because many heads have not been told by unions which teachers will turn up for work. Michael Gove, the Education Secretary, told MPs he would review the rules to force teachers to disclose this information in advance.

Some unions who are not on strike have told their members not to cross picket lines at school gates. That could deter teaching assistants meant to be filling the gaps left by striking members of the NUT and Association of Teachers and Lecturers.

Unions remain at odds with the Government over plans to raise the pension age, increase workers' pension contributions and to link pension values to the consumer prices index (CPI), a measure of inflation which is generally lower than the retail prices index (RPI).

Mr Cameron told the unions the strikes were " wrong – for you, for the people you serve, for the good of the country. It's the changes we propose that are right." Insisting that the Government's proposals were a "good deal" on pensions, he told the Local Government Association conference in Birmingham that the balance between what public-sector employees paid into their pensions and what the taxpayer contributed was getting "massively out of kilter".

Mr Miliband issued his strongest condemnation of the strikes last night, after telling his Shadow Cabinet at its weekly meeting to sing from the same hymn sheet. At the weekend, frontbenchers Peter Hain and Sadiq Khan were less critical of the industrial action than the Labour leader.

Calling for both sides to "think again" and return to the negotiating table, Mr Miliband said: "Strikes are a sign of failure on both sides and Thursday's industrial action is a mistake ... I understand why teachers are so angry with the Government. But I urge them to think about whether causing disruption in the classroom will help people understand their arguments. You do not win public backing about pensions by inconveniencing the public – especially not while negotiations are ongoing."

However, the Labour leader also blamed the Government for acting in a "reckless and provocative manner".

Public opinion does not appear to be on the side of the strikers: a ComRes poll for The Independent this week found that people believed by 50 per cent to 32 per cent that the Government should ban public-sector strikes unless there has been a turnout of at least 50 per cent in the ballot.

Mr Gove's appeal to heads to encourage parents to volunteer to come in and help keep schools open appeared fall on deaf ears as headteachers' leaders expressed "grave concerns" about the idea. Russell Hobby, general secretary of the National Association of Head Teachers, said: "It is probably not unlawful but we would strongly advise our members not to accept voluntary help to cover for absent staff this Thursday. When qualified staff are present, the voluntary help of parents is a very welcome contribution to schools and something to be very much encouraged. However, where qualified staff are unable to supervise them, the presence of voluntary, temporary helpers can have very serious implications for the safety and well-being of pupils."

Universities and colleges will also be hit by strike action as the University and College Union joins the strike. The union said this would cause "significant disruption" to around 350 colleges and 75 universities.

Two organisations supplying emergency nannies to help parents out in times of crisis reported a significant increase in bookings. Ben Black, of Emergencychildcare.co.uk, said he had seen a fivefold increase in bookings. Amanda Coxen, of Tinies, said it had received a 50 per cent increase in enquiries.

David Frost, director general of the British Chambers of Commerce warned: "Large-scale industrial action will have a significant impact on business confidence and inward investment, which are both critical to the UK's economic recovery. For example, the disruption caused by teachers striking means many parents will miss a day of work to look after their children, resulting in lost wages and damaging businesses' productivity".

Public solidarity? It depends who you ask

Michaela Zein, 39, accountant, London

"I can imagine how tough it is for teachers working today and I completely understand their motivations for striking. However, we have to look at the bigger picture and the public sector needs to adapt. After the recession, the private sector has already made great cuts across the board and has cut pensions for all employees. Personally, I don't think I would strike as I feel that these cuts are tough but perhaps necessary."

Jamie Quinn, 31, sustainability manager, London

"Public-sector employees often get paid less than private-sector workers. Indeed, one of the main justifications for working in the public sector is the guarantee of a favourable pension after suffering a working life with a poor wage. After putting in all that time and effort setting up for retirement – suddenly the goalposts are moved. I would picket if I was a teacher but there is a limit to my sympathy for the teachers. In these tough economic times, you have to be honest, and judge whether a poorly paid job is better than no job at all. I believe the majority of people would rather take the job and not grumble."

Matthew Wailing, 33, IT consultant, Sussex

"On principle, I do not believe in striking as a whole. I understand their frustration but striking is not the answer and does not resolve anything. The private sector has been suffering for a long time, and it is only now that the public sector is beginning to feel the burden. I recognise that the proposal of a good pension plan can compensate for poor wages but savings need to be made."

Joanna Brown, 33, artist, Scotland

"I have not been following the press coverage that greatly. However, separate to this, I believe that teachers do a very hard job but deliver the goods constantly. Thus on that basis, the teachers deserve a good and suitable pension for their work."

John Warrender, 48, university lecturer, London

"I think it is a good idea for public-sector workers to go on strike because it is widely known that public-sector workers are paid much less than private-sector workers. The work as a teacher is extremely arduous and the level of expectation upon them is vast. If I was a woman aged over 50 who had saved up a good pension, I would be extremely annoyed. The idea of changing the rules halfway through the game seems extremely unfair. If I was teacher, I would strike with them and despair."

Stuart Linden, 37, director of a paper merchants, Hemel Hempstead

"The events planned on Thursday are quite an inconvenience to me. I have taken a day's holiday to look after my daughter as one of us has to look after her. Personally, I am not angry about the event and if I were a teacher, I would strike. However, it isn't really the solution for a problem like this and I do not know how their actions will solve it."

Alison Muskett, 29, unemployed, London

"Although I hold no strong view on the matter, I do not believe the strikes are the right course of action. I have been speaking to one of my friends, who is a teacher, and she says she will strike – not because she wants to but because she fears social exclusion if she does not. Strikes never truly achieve their goal. The problem is the children that will be affected by this may get disenchanted by it. I would not strike as there has to be another way and strikes are not the right way to go about anything."

Peter Cook, 67, retired, Cheshire