The Discovery Channel team rides in the countryside during a training session prior to the 94th edition of the Tour de France. The race starts today in London.

London ? Trying to pick the winner of this summer’s Tour de France is anyone’s guess, especially since many of cycling’s biggest stars are sidelined because of doping scandals.

When seven-time champion Lance Armstrong was racing, it was easy to pick him as favorite. On the eve of today’s 4.9-mile prologue, it was hard to predict who will be wearing the yellow jersey in Paris on July 29.

“As soon as we start the race, we will know more about who is good,” Team CSC rider Carlos Sastre said. “At this moment, I don’t know.”

Sastre, Alexandre Vinokourov, 2004 runner-up Andreas Kloeden, Levi Leipheimer, Alejandro Valverde, Oscar Pereiro, Cadel Evans, Denis Menchov, Vladimir Karpets and Christophe Moreau all are among the contenders. The 33-year-old Vinokourov finished third in 2003 and fifth in 2005. Pereiro was second last year, and will become the 2006 winner if Floyd Landis is stripped of his title.

Landis, 1997 champion Jan Ullrich and 2005 runner-up Ivan Basso have been scratched off the list of contenders.

Landis is appealing a positive test for synthetic testosterone during last year’s race. Ullrich and Basso were tied to a Spanish doping investigation called Operation Puerto that implicated more than 50 cyclists. Basso has been suspended for two years and admitted “attempted doping.” Ullrich retired, but has denied wrongdoing.

Also missing will be sprint ace Alessandro Petacchi, who failed a doping test at the Giro d’Italia. The 33-year-old Italian, who won four sprint stages at the 2004 Tour, returned a “non negative” test for the asthma drug salbutamol.

Cycling enthusiasts hope the 2007 winner will be clean, but know there will be doubts about any rider who survives a three-week ordeal of 2,120 miles that includes six mountain hikes, three summit finishes and two individual time trials.

“It is obviously essential for us that the rider who will be in the yellow jersey and who will lift his arms in triumph will be the winner of the Tour de France,” Tour director Christian Prudhomme said Friday. “And that he won’t be caught (cheating) after.”

One of this year’s riders is David Millar, who was banned for two years in 2004 and stripped of his 2003 world time-trial title after admitting to using EPO. He says cycling has no excuses left.

“I think we’re learning in the last month or so that we can’t move into the future without dealing with the past,” he said Friday. “We have a pretty serious past that we’re going to have to face up to.”

The race starts on the second anniversary of the terrorist attacks on London’s public transportation network that killed 52 people. The prologue begins in Whitehall and passes the Houses of Parliament and Buckingham Palace, looping through Hyde Park and doubling back to finish on The Mall.

On Sunday, the first stage of the Tour starts on The Mall and crosses the River Thames three times before heading east and then southeast, ending in Canterbury. Then riders head to the more familiar roads of France.

The last time the Tour came to Britain, in 1994, an estimated 2 million people crowded the route to watch. At least 4,500 officers from London’s Metropolitan Police will provide security, along with officers from other forces and a small contingent from the Garde Republicaine, the unit of the French gendarmerie.

“We have total confidence in the British authorities,” Prudhomme said. “We are in daily contact with them – that’s obvious. Scotland Yard is one of the most efficient intelligence services in the world, if not the most efficient.”

Protocol normally would have the defending champion start last. But with Landis out, Pereiro will be last down the ramp.