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Ministers have clashed with union chiefs over claims next week's public sector strikes could cost the UK £500m and lead to job losses.

Downing Street has stood by the claim, saying it is a "fact" that closed schools will stop many parents going to work, which has an impact on "output".

But unions have accused them of scaremongering and "fantasy economics".

More than two million workers are expected to join action on 30 November over changes to public sector pensions.

Unions say proposals which require their members to work longer before collecting their pension and contribute more are unfair - but the government says change is needed to keep down the cost to the taxpayer, because people are living longer.

The Treasury made the £500m estimate based on lower public sector output and the knock-on effect on the private sector.

Cabinet Office Minister Francis Maude said at a briefing on Thursday: "If you lose a big chunk of output, it's hard to see how that doesn't translate into lost jobs."

However ministers could not put a figure on how many jobs could be lost and conceded that the economic impact would be no greater than that of a one-day holiday for the Queen's birthday.

Chief Secretary to the Treasury Danny Alexander said the £500m figure was a "reasonably worst case scenario" - based on the impact of all members of the unions who balloted over the strike not turning up to work.

It took into account the effect on the private sector, such as parents who could not go to work because their children's schools were closed.

"That is a loss we can ill afford at what is a very very difficult time for our economy overall."

He urged union members to look at the deal the government is offering to see how their pensions would be affected - and said he believed a resolution that would be a good deal to both public sector workers and the taxpayer was "within our grasp".

The prime minister's official spokesman said: "There is no question the strikes will have an impact. It is not costless, if lots of people can't go to work because they have to look after kids, that has an impact on output. That is a fact."

But the figures were immediately questioned by TUC general secretary Brendan Barber: "While the strike on November 30 will obviously cause disruption, the figures suggested by ministers are fantasy economics."

Mark Serwotka, general secretary of the Public and Commercial Services Union, told the ´óÏó´«Ã½: "They should settle the dispute, not go round with scare stories about financial calculations that they really should have thought of a long time ago."

And Gail Cartmail, assistant general secretary of Unite, accused the government of "plucking figures out of the air to justify its refusal to negotiate constructively with the unions".

The ´óÏó´«Ã½ reported on Wednesday that civil servants from across Whitehall were being lined up to fill in for border staff at UK ports and airports on the day, amid fears of major disruption for travellers.

On a visit to a Toyota car plant in Derbyshire, Prime Minister David Cameron said the government would "do what we can" to ensure airports remained open and "border queues are not intolerable".

But he said: "Everyone should be clear there is going to be disruption and the reason for that disruption, the responsibility for that disruption, lies squarely with the trade union leaders who've decided on a strike even while negotiations are on-going.

"I think that is irresponsible, I think it is wrong and people should know who to blame."

Labour leader Ed Miliband, who was heckled at the TUC conference for not backing the strikes and has been attacked by Conservatives for refusing to "condemn" them, said he was "very concerned about the costs of these strikes".

He said movement was needed on both sides, adding: "I thought it was a disgrace that the prime minister yesterday declared strikes were going to go ahead in a week's time. He should be spending the next week negotiating to stop these strikes happening."

He said the trade unions had to accept some changes were needed to public sector pensions - but that the government must realise that its conduct was "no way to conduct a negotiation".

Ministers say they expect two-thirds of schools to close, and although clinical and medical staff in hospitals will work - the British Medical Association and Royal College of Nursing are not taking industrial action - some staff in hospitals will be on strike which would cause some delays.

But they said they would not be calling on the Army to step in and were not expecting emergency services to be hit.

Earlier this month the government came forward with a revised pensions offer - which would guarantee no-one within 10 years of retirement would have to work longer or see their pension income fall and includes the promise of more generous rates of accrual - the rate at which the value of a pension builds up.

On Thursday Mr Alexander said there was "no more money on the table" to settle the dispute and the government had made a "very good offer". There have been suggestions that revised offer could be withdrawn altogether if no deal is agreed by the end of the year.