By Martha Buckley BBC News

The Tour de France will take in London's famous sights

France's biggest sporting spectacle, the Tour de France, comes to Britain for the third time this weekend, with the race's prestigious start - le Grand Depart - being hosted by London.

After the Prologue has taken in the sights of central London on Saturday, the tour proper will head off through Kent, with the first leg ending against the backdrop of Canterbury Cathedral on Sunday.

Hosting the start or end of any of the Tour's 20 stages is considered a great honour by the French, but the Grand Depart is considered to be the most prestigious of all.

Cycling fever has taken over London's parks

This year will be the first time the race has begun in such a major city as London - Strasbourg, where it began last year, being a more typical venue.

Organisers of events in London are hoping to take full advantage of the opportunity to showcase the city in the run-up to the 2012 Olympics.

In France, competition to host the tour is intense, with every town eager to bask in the glory of the race.

The bidding process involves much schmoozing, with hopeful mayors vying to wine and dine race officials.

In London's case, race organiser the ASO appears to have jumped at the chance to stage the Depart before a backdrop of world-famous landmarks in both London and Canterbury.

There was also a �1.5m fee paid by the London bid team - a group including Transport for London, British Cycling, UK Sport and Sport England - to the ASO.

Staging the event itself is costing another �5.3m, but London and Canterbury are expected to receive a massive economic boost from tens of thousands of visitors.

Spectators wait for hours to see the riders go past in a flash

In total, the tour is expected to be worth �70m to the UK economy.

Hosting the Tour is a massive undertaking. Two million people are expected to line the routes of the first leg through London and Kent and hundreds of road closures are planned.

In France local authorities pull out all the stops to make their visitors welcome by putting on festivities and sprucing up their towns - even those where the race will merely flash by in a few minutes can expect hundreds of visitors.

Catherine Payronnet, of the tourist office in Gap, which hosted the start of a stage in 2006, says the population of the town, normally 40,000, at least doubled.

Spectators get in carnival spirit, dressing up to wave on the cyclists

She said: "There were people everywhere, most at the start and finish but also all along the route where the Tour was to pass. People arrived days beforehand with their caravans to get a good spot. It was like a big party."

Hosting the Tour can put even the smallest town on the map - with TV coverage reaching a potential audience of 2.5 billion in more than 80 countries, it is publicity beyond the wildest dreams of cash-strapped local authorities.

In London, organisers are gearing up to make the most of the party atmosphere with a whole string of festivals, carnivals, concerts, street parties, cycling events, markets, displays and exhibitions planned across the city.

More festivities will greet visitors in Canterbury, whose 35,000 population is expected to more than double.

French villagers find a way to make sure their village gets noticed

Along the route, towns and villages are being decorated to welcome the race, with sections of the route lined with miles of bunting and festivities planned from barn dances and carnivals to cream teas and village fetes.

As well as their 15 minutes in the spotlight, these places are all hoping for a share of the �37m boost the race is expected to bring to Kent. It is all a far cry from the tour's first trip to the UK in 1974, when the riders were confined to a circuit of dual carriageway near Plymouth.

Britain was better prepared in 1994, when it hosted two stages of the tour - from Dover to Brighton, then Brighton to Portsmouth - attracting three million spectators.

I just hope the crowds are going to stay on behind and spend some money Dennis Knight Send us your comments

However, the 1994 experience looks set to pale into significance compared with the huge crowds expected this weekend.

The picturesque Weald village of Goudhurst, for example, hosted just a handful of spectators in 1994 - this year it is planning for 10,000.

Virtually every pub, restaurant, shop and hall in the village will open, serving refreshments from 0800 BST on race day, with beer tents, children's entertainment and barbecues laid on and fields converted to car parks.

Dennis Knight, chairman of Goudhurst's Tour de France steering committee, says: "There's no comparison at all with 1994. I just hope the crowds are going to stay on behind and spend some money. And I hope to goodness it doesn't bucket it down with rain."