The biggest strike for five years will cause huge disruption to schools, courts and travel tomorrow in the most serious industrial challenge to the coalition since it was formed.
Hundreds of thousands of teachers, lecturers, civil servants and other workers will walk out for 24 hours in protest at controversial plans to change their pensions, cut jobs and freeze pay.
Mark Serwotka, leader of the Public and Commercial Services union, said it was the most important strike in his union's history, adding: "Everything we have ever worked for is under attack."
The Prime Minister launched a fresh attack on the strike, while business leaders warned it will have a "significant impact" on industry.
David Cameron told MPs there was no case for industrial action and repeated that the proposed changes to public sector pensions were "fair".
Speaking at Prime Minister's question time, he said he hoped "as many mums and dads as possible" would be able to take their children to school despite the action.
"I don't believe there is any case for industrial action tomorrow, not least because talks are still ongoing. It is only a minority of unions who have taken the decision to go ahead and strike.
"What we are proposing is fair. It is fair to taxpayers but it is also fair to the public sector because we want to continue strong public sector pensions."
He also attacked Labour leader Ed Miliband, who did not ask about the walkout, accusing him of being "in the pocket of the unions".
The British Chambers of Commerce said disruption will lead to many parents having to take the day off work to look after their children, losing them pay and hitting productivity.
Paul Griffin of law firm DBS Paul Griffin warned that workers could face disciplinary action if they take time off to look after their children.
He said: "Anyone thinking of phoning into the office tomorrow and claiming absence on the grounds of a parental emergency could find themselves in hot water. Whilst parents are entitled to unpaid time off to arrange childcare due to sickness or some other such unpredictable event, these strikes have been known about for months.
"Employers could argue that this situation does not qualify as an emergency and take disciplinary action against individuals who don't turn up for work."
Picket lines will be mounted outside school gates, courts, jobcentres, Parliament, driving test centres and Government buildings by members of the National Union of Teachers (NUT), Association of Teachers and Lecturers (ATL), University and College Union, and PCS.
Police leave has been cancelled in London, where union leaders and thousands of activists will take part in a march, followed by a rally in Westminster.
Business Secretary Vince Cable said he believed the public would find it difficult to understand why the strike was going ahead when negotiations over pensions were continuing.
It was "imperative" that public sector pensions were reformed, said Dr Cable, adding that he intended to be in his office tomorrow and hoped that if any Business Department staff went on strike it would not "spoil" the working relationship.
Mike Cherry, policy chairman of the Federation of Small Businesses, said: "Small firms are concerned about the impact that the proposed strike action will have on their ability to do business on Thursday.
"The recovery remains in a fragile state and this action will have a wider impact on the economy as businesses lose productivity - something the economy simply cannot afford."
Airports have warned travellers to expect disruption on arrival into the UK as immigration and customs officers join the strike.
Passengers have been advised to consider travelling on a different day to avoid delays as hundreds of staff walk out.
A spokeswoman for Virgin Atlantic said: "Virgin Atlantic is working closely with the UK Border Agency to support their contingency planning and minimise disruption to our customers.
"We will keep our passengers informed of the situation through text messages, advice at check-in and onboard announcements."
High-speed Channel Tunnel train company Eurostar said it would not be affected by industrial action.
It added that it was running two extra trains tomorrow - one from London to Paris, the other from Paris to London, to cope with expected extra demand.
NUT leader Christine Blower said: "The action tomorrow is regrettable but unfortunately, due to the position that the Government has taken, unavoidable. Teachers have been left with no option but to demonstrate the strength of feeling they have about the unnecessary proposals, which will see teachers paying more, working longer and getting less for their pensions.
"The National Audit Office has confirmed that teachers' pensions are affordable. The Institute of Fiscal Studies has said all the evidence demonstrates that the costs of public sector pensions are falling. It is increasingly obvious that the Government is pressing ahead with changes on an ideological, not economic, argument."
Mr Serwotka urged union members to ignore Government calls not to strike, adding: "Thousands of jobs are at stake, lower pensions are set to cost three times as much, and pay is frozen while inflation soars.
"If you have never taken part in strike action before, make tomorrow's national action your first."
TUC general secretary Brendan Barber will visit picket lines in the South West today and speak at a rally in Exeter where he will say: "The brutal truth is simply this - the burden of deficit reduction is being piled unfairly on to millions of low and medium-paid public sector workers who did nothing to cause the crash.
"Their pay has already been frozen for two years, even though inflation is higher than it has been for over a decade.
"Meanwhile, those who are actually guilty of causing the crash in the finance sector are busy getting back to business and bonuses as usual, escaping the scene of their crimes just as a hit-and-run driver would flee a car crash. This is a gold standard for unfairness."
UCU general secretary Sally Hunt will tell the London rally: "I come from a family of teachers, and I could never underestimate the power of education to change things or the determination of teachers to stand up for what is fair. An attack on teachers is an attack on education itself, and an attack on education is an attack on hope.
"I received an email from Diane from Norfolk who, after 30 years teaching, is on a pension of £120 a week. So to hear Nick Clegg talk of public sector fat cats and gold-plated pensions makes my blood boil. He must apologise for that remark.
"The average pension of a female college lecturer is just £6,000 a year. This is a Government that has already presided over an increase in the income of the richest 1,000 people by 18%. How dare they call us gold-plated? How dare they to preach to us about fairness?"
A survey of 1,000 public sector workers by recruitment consultants Badenoch & Clark showed that a third believed pensions were worth striking over, although 61% said industrial action will not make a positive difference to the dispute.
Managing director Nicola Linkleter said: "Our research over the past year has consistently highlighted morale as being low in the public sector. Not only have workers had a lack of job security, reduced resources and longer working hours, but their benefits packages are now under scrutiny too."
Downing Street said the Government was doing everything possible to ensure it was "business as usual" tomorrow.
"We have been for some time putting contingency plans in place to ensure essential services are maintained," the Prime Minister's official spokesman said.
Based on returns from around three-quarters of the schools in England, the Government expects around a third will be shut completely, a third will be partially affected, and a third will remain open.
Officials expect that around one in five of the UK's 500,000 civil servants will take part in the strike.
Nevertheless, Downing Street said it was thought the "vast majority" of courts, job centres, and HM Revenue and Customs call centres would remain open as usual.
Management are being redeployed to operate border security checkpoints, which are also expected to stay open, although the Prime Minister's spokesman acknowledged that delays were "possible".
The spokesman said that in some offices, civil servants who are affected by the teachers' strike were being allowed to bring their children to work to minimise disruption to services.
Cabinet Office Minister Francis Maude said: "Strike action is unnecessary and premature while discussions - set up at the request of the TUC - are ongoing. The majority of civil servants and teachers themselves did not vote for this action, showing how extremely limited support is for this strike.
"The Government has always been committed to working openly and constructively with the TUC on public sector pensions and we believe both sides have a responsibility to see the talks through.
"We can assure the public that we have rigorous contingency plans in place to ensure that their essential services are maintained during the strike action on Thursday. We will do all we can to make sure it is business as usual tomorrow and that people can continue to claim their benefits, pay their pensions and access job centre websites.
"Less than 10% of the Civil Service workforce has voted for strike action and only about a third of teachers, which shows the real lack of support for the kind of widespread action union leaders are hoping for."