Chris Froome faces tough ride for Tour hat-trick

"This is the toughest challenge I've faced in my career, to target a third Tour de France," Froome, who prevailed on the French roads in 2013 and 2015, said.

The 31-year-old will hope that his performances are not scrutinised with the same negativity as last year when he faced a barrage of doping insinuations after smashing his rivals in the first mountain stage.

Team Sky's Froome, who won the Criterium du Dauphine warm-up race earlier this month, proved last year that he could be impervious to outside pressure, hanging on to his lead despite the media attacks and those coming from the crowd, which included having urine thrown at him during one stage.

A race that never ceases to throw up controversy will also be keen to stamp out rumours of illegal mechanical assistance.

It is a threat so serious that organisers and the International Cycling Union (UCI) announced they would use thermal cameras to detect and deter hidden motors.

To help Froome succeed, Sky have assembled arguably the strongest squad around their leader, with Geraint Thomas, Mikel Landa, Sergio Henao and Wout Poels earmarked to protect him in the mountains.

"It's not just about beating Froome. It's about beating the whole Team Sky," Alberto Contador (Tinkoff), also gunning for the third Tour title that would cement his status as the greatest grand tour rider of his generation, told L'Equipe.

For the Spaniard, who won the Tour in 2007 and 2009, there will also be the small matter of beating Colombian Nairo Quintana (Movistar), the best pure climber in the world.

"This will be the best version of Nairo," Movistar team manager Eusebio Unzue said. "He's now more confident, more prepared, with a higher level of experience.

"Now he is ready to win the Tour (after finishing runner-up to Froome in 2013 and 2015).

Thibaut Pinot (FDJ) will mount the French challenge after a strong season. He is one of the big guns who will be hoping to fare well in the 37.5-km undulating time-trial in the 13th stage where he can gain time on Contador and Quintana.

Some believe the biggest threat to Froome could be from a collision although Team Sky principal Dave Brailsford said he is not too worried about the prospect.

"If you worry about people ganging up on you, you are not able to focus on yourself. We'll make the race and choose our tactics accordingly. It's about being proactive and controlling, rather than worrying about other people," he said.

Other contenders include 2014 champion Vincenzo Nibali and his Astana team mate and fellow Italian Fabio Aru as well as Quintana's team mate Alejandro Valverde and former Froome lieutenant Richie Porte of Australia (BMC).

All of them have probably targeted the Queen stage finishing up the Mont Ventoux in the 12th stage.

World champion Peter Sagan (Tinkoff) will be the hot favourite to claim a fifth green jersey for the points classification and move one shy of Erik Zabel's record, while the bunch sprints should be an all-German battle between Marcel Kittel (Etixx-Quick Step) and Andre Greipel (Lotto-Soudal).

Security will be tight with about 23,000 police deployed and special forces ready to intervene amid concerns the race could be targeted after 130 were killed in last November's coordinated Islamic attacks on Paris.