Tour de France prologue, 1500 BST
Live coverage on BBC Sport website
BBC Radio Five Live start coverage from 1600 BST
London will welcome a million visitors when it hosts the prologue and opening day of the Tour de France this weekend.
The 2007 Tour de France begins in London on Saturday
Some 189 riders will compete in a 7.9km time trial on Saturday as the famous race makes its third visit to Britain.
The winner will wear the leader's yellow jersey as the peloton rides 203km to Canterbury in Kent on Sunday.
Mayor Ken Livingstone said the tour's Grand Depart is "the biggest sporting event hosted by London ahead of the 2012 Olympics".
The Tour then crosses to Belgium before moving into France, with the race ending in Paris on 29 July.
The 3,570km route features 20 stages, with six days in the mountains, including three mountain-top finishes and two time trials.
The Tour, which has started in a host of different countries in recent years, made its debut in Britain in Plymouth in 1974 and returned to Dover, Brighton and Portsmouth in 1994.
I want the guy who raises his arms in triumph on the Champs-Elysees to be the irreproachable champion
The event is watched by as many as 15 million people on the roadside every year with an estimated two billion watching on television over the three weeks.
But the 94th edition of the race, won by American Lance Armstrong for seven years in a row from 1999 to 2005, will begin under a cloud as drug allegations again haunt the sport.
Floyd Landis, last year's winner, is awaiting the verdict of a doping hearing, while Giro D'Italia champion Danilo di Luca is also being investigated.
In addition, 2006 Giro winner Ivan Basso was recently handed a two-year ban and 1996 Tour de France winner Bjarne Riis admitted using performance-enhancing drugs.
Organisers are desperate to crack down on drugs in cycling and have asked teams to sign a pre-Tour pledge.
Millar (left) and Wiggins will fight it out in Saturday's prologue
The charter stipulates that all riders must give DNA samples to Spanish authorities investigating the Puerto affair 15 months after a blood-doping scandal erupted in Spain.
And it requires them to lodge a year's salary which will be forfeited if they are convicted of any doping charges.
Every rider on the start list had signed up by Friday, while on Thursday the entire field was blood tested to the satisfaction of organisers.
"I want the guy who raises his arms in triumph on the Champs-Elysees to be, and remain, the irreproachable champion," said race chief Christian Prudhomme.
Britain will be represented by David Millar, Mark Cavendish, Geraint Thomas, Charlie Wegelius and Bradley Wiggins - the biggest British contingent for 20 years.
Thirty-year-old Millar, who is riding his second Tour since serving a two-year doping ban, is one of only four Britons - along with Tom Simpson, Chris Boardman and Sean Yates - to wear the yellow jersey after his win in the 2000 prologue.
Now an ardent anti-drugs campaigner, the Scot is one of the favourites for Saturday's prologue, along with 27-year-old Londoner Wiggins, the world and Olympic pursuit champion.
The important message is that London is open for business as usual
Met Police chief Ian Chappell
Overall race favourites include Kazakh rider Alexandre Vinokourov, German Andreas Kloden, Spanish duo Alejandro Valverde and Carlos Sastre, and American Levi Leipheimer.
Security has been stepped up in London, with 5,000 police officers set to be deployed, following the failed terror attacks in Britain last weekend.
"The important message is that London is open for business as usual, but we have reviewed our security plans in light of what happened," said Superintendent Ian Chappell of London's Metropolitan Police.
Eight suspects have been arrested in Britain and Australia over two botched car bomb attacks in London on 29 June and a third at Glasgow airport the following day.