Jonathan Sexton has been ruled out of Ireland's opening RBS Six Nations Championship match against Scotland on Saturday due to a calf injury, the Irish Rugby Football Union has announced.

Flanker Peter O'Mahony will also miss the Scotland clash after suffering a hamstring problem, in another blow to head coach Joe Schmidt's side.

Uncapped Munster centre Rory Scannell is Ireland's only other recognised option at fly-half, after Schmidt chose to overlook Bordeaux's Ian Madigan.

The Irish Rugby Football Union (IRFU) confirmed the sizeable injury blow by tweeting: "Johnny Sexton has done some running but will not be considered for selection for this weekend.

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

Sexton suffered his latest injury blow in Leinster's 24-24 European Champions Cup draw at Castres on January 20.

The 31-year-old limped out of the first half of Leinster's draw in France, detecting the calf issue himself with his provincial bosses hoping a quick withdrawal would boost his chances of being fit to start the Six Nations.

Ireland had expected Sexton to return to full training on Tuesday, and had banked on the 63-cap playmaker winning his fitness battle to face the Scots.

Now Ulster's 25-year-old Jackson will step up into Ireland's number 10 shirt, having run the backline operations throughout training last week.

Schmidt has already defended his decision to overlook former Leinster fly-half Madigan for selection.

Ulster's Paddy Jackson will step up to replace the injured Sexton (Getty)

The IRFU favour those playing club rugby in Ireland for Test selection, with Schmidt happy to follow that trend.

While 19-cap Jackson has the experience to cope with the starting jersey, Keatley boasts just four caps - and has not featured since Ireland's 26-3 win in Italy in February 2015.