LONDON, Aug 29 (Reuters) - Thousands of British prison officers launched a surprise 24-hour strike on Wednesday, their first ever national stoppage.

The Prison Officers' Association (POA), which has previously adhered to a no-strike agreement, said its members working at more than 140 jails in England and Wales had walked out in a dispute over pay.

There have been signs that Prime Minister Gordon Brown's government could encounter a difficult winter, with other public sector workers already voicing dissatisfaction with their wages.

But union sources say the chances of widespread strikes had fallen recently as the government moved to improve wage deals and avert industrial action that could prove electorally disastrous.

It was not clear what effect the strike was having at the prisons, which are almost full to capacity with around 80,000 inmates, but the government said it had contingency measures to ensure security and there was no risk to the public.

It said the strike was illegal and "wholly unjustifiable".

Since Brown took over from Tony Blair in June, expectations of an early election this year have increased because of polls showing he has a commanding lead over the opposition Conservatives.

Brown has consistently repeated his commitment to keep public sector pay rises in line with the government's inflation target of 2 percent, but economists argue that position is proving untenable.

"Combined with the steady acceleration in the cost of living, the risks to wage growth are all in one direction -- up," said Alan Clarke, an economist at BNP Paribas.

The POA said it had taken action because the government had decided to implement a 2.5 percent pay increase in stages, which it argued amounted to just a 1.9 percent rise for its members.

Britain's prisoner numbers have reached record levels in recent years, leading to some criminals being released early to ease overcrowding and accusations that the government was mismanaging the criminal justice system.

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