The 31-year-old, a veteran of 56 Tests, was expected to provide bench cover for Ireland captain Rory Best during European rugby union's flagship tournament.

Cronin, however, suffered the hamstring tear against Italian side Zebre and is out for more than two months, with Ireland starting their Six Nations title bid away to Scotland in Edinburgh on February 4.

"It's quite a significant hamstring injury," Leinster coach Stuart Lancaster told rbs6nations.com on Monday.

"It's a real shame for him as he's actually playing great rugby but it's a good opportunity for us to get him right and make sure we don't get a recurrence of this because it must be hugely frustrating for him."

Former England coach Lancaster added: "He's got himself into great form and now he'll miss the RBS 6 Nations.

"He went to push-off and accelerate around the corner and saw it go. He's deeply disappointed but he'll be back a better player."