There is a 30% chance of a white Christmas, Mr Hill said

It is fairly nondescript, with only a couple of rain gauges and a barrier skirting its perimeter.

But this section holds the key to whether or not London will have a white Christmas.

If it snows within this area on 25 December, the Met Office will officially declare a white Christmas for the capital. If not, the wait will continue for another year.

Every hour on the hour, one of my forecasters has to wander up and make sure it hasn't snowed yet

Alex Hill

Met Office

White Christmas for Scots

But for the Met Office staff responsible for making the crucial call, it will be just another day at the office.

"All throughout Christmas Day, every hour on the hour, one of my forecasters has to wander up and make sure it hasn't snowed yet," said Alex Hill, the head of the London Met Office.

"There's a chance any showers here could miss us completely but for this little square I'd say there's a 30 per cent chance it'll see a shower on the day."

Scotland, Northern Ireland and northwest England were likely to see snow in the run-up to Christmas, the Met Office said on Wednesday.

Strong odds

With wintry weather coming in, snow could not be ruled out anywhere by Christmas day, forecasters said.

Christmas day snow showers were expected in north western Scotland, Northern Ireland, parts of England and Wales.

Bookmakers have been slashing their white Christmas odds all week, with snow in Glasgow and Aberdeen odds-on, and odds for London and Cardiff constantly shorterning too.

Scotland is almost certain to see snow for Christmas

Each city has its own designated areas where snow must fall for the bookies to pay. In Aberdeen it is the airport, in Glasgow it is the office of the Daily Record newspaper while the Met Office again performs the duties in Cardiff.

But punters who may be growing nervous about the possibility that snow could cover a city but somehow miss the vital measuring area can take heart - the Met sometimes bends the rules.

"We would probably still call it if there was snow down the road but nothing could be detected within the actual square," Mr Hill said.

"It doesn't necessarily have to lie [on the ground], all it has to do is fall. And it has to be spotted as falling."

The leading bookmakers say they are happy with this less-than-stringent interpretation.

"Four years ago it snowed a few hundred yards away from the office and we still paid out," said William Hill spokesman Rupert Adams.

"It's designed as a bit of fun, we hope it allows lots of people to have a fiver on it and about 95% back it to snow purely because everyone wants it to snow."

'Disappointment'

Ladbrokes' William Lush said those punters may be disappointed - even if they win - if they have grand plans of Christmas Day snowball fights and snowmen.

Snow in Cambs in November signalled a cold Christmas

With both bookies expecting a hefty payout, it would appear the only people unable to capitalise are the ones best placed to judge.

"I'm not allowed to bet on it," the Met's Mr Hill said. "It would be a bit unfair really wouldn't it?"

"As a civil servant we're not allowed to profit on what is essentially government information."

For those who can risk a flutter, the weather watching for 2005 could begin while this year's leftover turkey still stocks the fridge.

"The book opens on Boxing Day for next year," Mr Adams said.