Fly-half Jonathan Sexton had recovered from a hamstring injury but picked up a fresh ankle injury in training © Getty Images

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Players/Officials: Johnny Sexton

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Teams: British and Irish Lions | Ireland

Ireland fly-half Jonathan Sexton has been ruled out for up to six weeks with a foot injury - casting doubt over his participation in the British & Irish Lions tour to Australia this summer.

Sexton damaged a tendon in his left foot at the end of training on Thursday, resulting in his withdrawal from the starting XV for Saturday's Six Nations clash with Italy at the Stadio Olimpico.

The 27-year-old had only just recovered from the hamstring injury he sustained against England on February 10 and now faces an even longer spell on the sidelines.

"We were told yesterday that Jonny will be out for four six to weeks," Ireland coach Anthony Foley said.

The prognosis means Sexton will almost certainly miss Leinster's Amlin Challenge Cup quarter-final against Wasps on April 5, leaving him with just two RaboDirect PRO12 fixtures to prove his fitness to Lions selectors.

Lions head coach Warren Gatland is scheduled to announce his squad on April 30, so any setback in his recovery could threaten his involvement with the tour to Australia.

The Leinster playmaker has only completed 120 minutes of Test rugby this year, but counting in his favour is his position alongside England's Owen Farrell as one of the two best fly-halves available to Gatland.

"That was a doctor saying four to six weeks, it wouldn't surprise me if Jonny is two to four weeks away," Foley said. "You take into account his powers of recovery from his hamstring tear and how quickly he came back from that, combined with his attitude to rehab, and he could be back sooner.

"Having worked with the lad this season, I imagine when he gets into the thick of the rehab he'll fly through it, although it will be tough mentally at the start."

Just two hours after being selected at fly-half against Italy on Thursday lunchtime, Sexton was replaced by Paddy Jackson when the extent of his injury was discovered.

"At the end of yesterday's session, Jonny pulled up and none of us thought much of it. It was only afterwards that we found out he'd torn a ligament," Foley said. "We feel for the lad because he's worked very hard over the previous three weeks to get back from the hamstring injury. To fall down on the last session before we boarded the flight for Rome is tough. We wish him all the best in his recovery."

Ireland's Six Nations campaign has been undermined by a staggering number of injuries, but Foley insists morale remains high.

"The weeks have flown by and if we had to do another eight weeks we'd do it in a heartbeat," the former Munster No.8 said. "At times the results haven't gone our way, but that hasn't been for want of effort, attitude or the way we've gone about our job. It's been exemplary.

"It doesn't feel like the end of the competition because everything seems very fresh and enthusiastic. When you have the number of injuries we've had, it's just about getting on with the job.

"You're dealing with people and their careers, but your sole focus is getting the team right for tomorrow. That might sound blunt, but that's the way it has to be run."