Factbox - National strike over pensions

(Reuters) - Britain faces its biggest one-day national strike in 30 years on Wednesday over pension reforms which have enraged public sector workers.

The co-ordinated mass action comes against a gloomy backdrop of faltering growth, rising unemployment, big public-sector layoffs and wage freezes as the government pursues an austerity drive designed to slash a record budget deficit.

Here are some facts about the strike and its likely impact:

* A coalition of 30 trade unions, representing some 2 million workers, is expected to take part.

* Workers including teachers, probation officers, police support staff, civil servants, cleaners, paramedics and some nurses, as well as refuse and tax collectors and border agency personnel are taking part. Government-paid physicists, botanists and museum curators are also expected to join in.

* Some unions involved have not taken action in more than a decade. One union, representing head teachers, will be striking for the first time in its 114-year history

* Unison, the biggest union involved, says numbers may match those of the nine-day 1926 General Strike, when more than 2 million workers took part.

LIKELY IMPACT

* The strike could force two thirds of schools to close. Prime Minister David Cameron has suggested employers should allow parents to take their children to work to beat the stoppage.

* The government says the strike will cost the economy 500 million pounds, in lost output a figure unions have called "fantasy economics." ? * Unions are planning 1,000 demonstrations and rallies across the country. Picket lines are expected to spring up in workplaces including schools, job centres, town halls, libraries, hospitals and health clinics.

* Few transport unions are involved but ports and airports, like Heathrow, Europe's busiest, are likely to be severely disrupted as border control staff walk out.

* The government is flying home British embassy staff to help out with border checks.

* Airlines are cutting flights into Heathrow because of fears of long delays and overcrowding.

PENSION REFORMS

* An overhaul of pensions is part of the Conservative-Liberal Democrat coalition government's package of austerity measures to tackle a budget deficit that peaked at 11 percent of national output when it took power.

* The government insists that public sector workers increase contributions they pay into state-controlled pension schemes to make annual savings of 2.8 billion pounds ($4.35 bln) by 2014.

* It says pension reform is vital as the system is no longer affordable, partly because of longer life expectancy. It says public sector pensions are more generous than in the private sector, which is unfair to taxpayers.

* Workers across the public sector will be forced to contribute an average of 3.2 per cent more annually to their pension funds.

* Ministers say the normal pension age should be linked to the state pension age, meaning many public sector staff would have to work longer, as the government is planning to raise the statutory retirement age to 68 from 2020.

* The government's decision to switch to the lower consumer prices index (CPI) from the more generous retail prices index (RPI) when calculating inflation adjustments has already reduced the value of some pensions by 15 percent.

* Unions say the reforms would mean people would work longer and pay more for worse pensions.

* The government, which wants workers to receive pensions based on average earnings rather than final salaries, made a revised offer earlier this month. It was not accepted by the unions.

MORE STRIKES?

* Unions have warned of more rolling strikes next year if the government fails to make a new significant offer by the end of the year, a deadline for negotiations. The government says no more money is available and has threatened to withdraw a revised "generous" offer it made in early November if workers strike.

* Union strength is concentrated in the public sector and estimates of workers involved in the walkout only represent less than 10 percent of the total British workforce.

* Union membership has shrunk in Britain since Margaret Thatcher's Conservative government took on the unions. Membership fell sharply through the 1980s from 56 percent of the workforce at the start of that decade to around a quarter by the end of the 1990s.

(Reporting by Stefano Ambrogi)