Travellers arriving in Britain next week will have their passports checked by temporary staff drafted in by the government, as border guards join up to two million people staging a strike over public sector pension reform.

Thousands of schools across the country will also close in what is expected to be the biggest union protest since the Winter of Discontent in 1979 that helped Conservative leader Margaret Thatcher sweep to power.

The strikes are a fresh challenge to the current Conservative-led coalition which took power 18 months ago and made cutting a record budget deficit its priority.

The government is expected to say next Tuesday that weak economic growth means it will miss its target of eliminating the deficit by the time of the next election in 2015, prolonging an era of austerity.

Treasury Minister Danny Alexander said the strikes could cost the British economy over 500 million pounds in lost output.

That's a loss which we can ill-afford, at what is a very difficult time for our economy overall, Alexander said.

We are putting in place contingency plans, we are looking at ways in which we can find additional staff for our borders, for example, to ensure that as much as possible we can maintain services, Alexander said.

Workers in several European countries have protested over pension reform as cash-strapped governments try to cut the cost of supporting ageing populations.

Britain hopes to make annual savings of 2.8 billion pounds by 2014 through higher contributions from workers, many of whom are already facing a wage freeze.

Staff based at British embassies were being flown home to help out with border checks and other volunteers were being drafted in from across government departments.

Workers from outsourcing firm Serco, which runs some of Britain's prisons, were also being trained to replace striking border agency guards, according to a British media report. The FTSE 100 listed company would not comment when contacted by Reuters.

Border security is a particularly sensitive issue in island nation Britain at present after the government admitted this month it did not know how many suspected terrorists or serious criminals had entered the country after officials relaxed passport controls to shorten immigration queues.

Unions condemned the government response to the strike.

Instead of scratching around trying to put untrained people on the front line, they should say today they want urgent talks to try and resolve the dispute, said Mark Serwotka, head of the PCS union which represents 290,000 state workers.

(Additional reporting by Mohammad Abbas and Tim Castle, editing by Mark Heinrich)