Ireland fly-half Johnny Sexton has been ruled out of his side's Six Nations opener with Scotland due to a calf injury.

Sexton has been struggling with the problem since Leinster's European Champions Cup draw with Castres earlier this month and has failed to recover in time, meaning that Ulster's Paddy Jackson is likely to come into the side.

Ireland will also be without flanker Peter O'Mahony for the match due to a hamstring injury, but both could return in time to face Italy on February 11.

"Johnny Sexton has done some running but will not be considered for selection for this weekend," read a tweet from the IRFU.

"Johnny did some light running but is still feeling some tightness. He will continue to rehab but won't be selected this weekend."

Ireland have won the Six Nations title in two of the previous three years.