London is officially enjoying a white Christmas after a few flakes of snow fell on the roof of the Met Office weather centre.

Although the city's streets still look grey and damp, and thousands on England's south coast are mopping up from overnight flooding, the Met Office's word is the official definition of the phenomenon.

It will be enough to cost bookmakers - who until two days ago were still offering up to 50-1 on the chances of a white Christmas - hundreds of thousands of pounds.

William Hill spokesman Graham Sharpe said: "Earlier this week forecasters told me snow was about as likely as aliens landing on the Millennium Dome. That's the last time I listen to them."

He said the company was now due to pay out some £50,000 for those who bet on a white Christmas in the capital - with more to follow from other regional bets if snow was confirmed in other cities around the UK.

Compounding the misery bookies are also facing a "double whammy", Mr Sharpe said, as many punters had also staked money on a wet Christmas.

The white Christmas is London's first since 1996, although one has to go back as far as 1895 to find the last Christmas-card blanket of snow covering the city on Christmas morning.

Forecaster Graham Webster said experts had been caught out by the sudden deepening of a depression over the country, which had sucked cold air from Iceland and Greenland over the UK.

He said: "We didn't expect it, but the weather has been a bit wild all round this Christmas."