Defending Tour de France champion Chris Froome expressed his dismay after discovering his friend and former team-mate Daryl Impey had tested positive for drugs as the cycling world descended on Yorkshire for the Grand Depart.

Froome had to deal with brickbats and continual questions over his performances in winning the 100th Tour last summer, and his arrival in Yorkshire coincided with the news that Impey had tested positive for Probecenid after February's South African Championships.

Impey last summer became the first African to wear the Tour leader's yellow jersey, but will not start Saturday's first stage from Leeds to Harrogate after being stood down by his Orica-GreenEDGE team.

Kenya-born, South Africa-schooled Froome and Impey were team-mates at Barloworld in 2008, when Froome rode his first Tour, and the Team Sky leader was surprised at the second positive finding of the week, after Roman Kreuziger was made unavailable by Tinkoff-Saxo due to irregularities relating to his biological passport.

"It was quite shocking this morning to have the news about Daryl, someone I know really well," Froome said.

"That is massive news in cycling (and) it's always harder when you know the person on a personal level.

"I did send him a message when I found out. I just said I was really shocked to read the news. And I hope things get resolved.

"Obviously I don't really know the details of the case. I don't know if it is something that can be resolved on his side or if it is 100 per cent definite.

"Knowing Daryl's character - he has always been very outspoken against doping - so it is a big shock to me."

Last July, in the first Tour since Lance Armstrong was stripped of his record seven titles for doping, Froome faced constant questions over the legitimacy of his performances, but proclaimed from the top step of the Paris podium that his yellow jersey would "stand the test of time".

His Tour build-up has been overshadowed by insinuations based on a therapeutic user exemption certificate for asthma medication.

"For me personally, my integrity is extremely important to me," Froome added.

"With the TUE case that came out a few weeks ago, that was something I really wanted to clear up and tell people my side of events.

"I do think it's a good thing that we talk about it (doping), that we put all of our cards on the table and we tell people how it is now.

"It is a shame that with cycling's past we find ourselves in this situation now.

"But the only way we're going to move on from it is to accept what has happened. Get it all out there. And then move on - s how people this is not how it's done any more."

Sir Bradley Wiggins famously exploded with a foul-mouthed rant in 2012 when he was questioned about doping at the end of his first day in the yellow jersey.

Wiggins became the first British winner of the Tour in 2012, but has not been back to the race since, missing last year's race through injury before being omitted from the Team Sky squad this summer, much to the disappointment of many British supporters.

Froome and Wiggins have had their differences in the past and the Team Sky leader insists he had no say in whether his predecessor as Tour champion would be in the nine-man squad.

"Just to make it absolutely clear, I do not have a role in selection," said Froome, who will be supported by Richie Porte, Bernhard Eisel, Vasil Kiryienka, David Lopez, Mikel Nieve, Danny Pate, Geraint Thomas and Xabier Zandio in the next three weeks.

"I do speak to (Team Sky principal Sir) Dave Brailsford about generally the group of guys I've been racing with, but, if you look at how a Tour team is generally selected, the guys doing a Tour will be doing altitude camps together, racing together as a group."

Froome admitted Wiggins' selection would have had an impact.

He said: "It would definitely change the dynamic of the team having Bradley on the team, but that's bike racing."

Froome is fit and raring to go after a crash at last month's Criterium du Dauphine saw him relinquish the lead and eventually finish 12th.

He added: "I did take me a while to start feeling normal again on a bike after that crash.

"I took quite a big knock to my left hip and there was a bit of underlying bruising there.

"I'm confident that's behind me now."

Froome knows the first week, which also sees stages from York to Sheffield and Cambridge to London, will be challenging.

"Given Yorkshire's narrow, twisting, undulating roads on the first couple of stages, it's going to make for some pretty hairy racing," he said.