Dublin - Influential flanker Dan Leavy has been ruled out of the Rugby World Cup due to a knee injury, dealing a blow to Ireland and to Leinster's defence of their Champions Cup title.

Leinster said on Monday that a scan had revealed the 24-year-old had a "complex knee ligament injury".

He had been stretchered off only 11 minutes after coming on as a replacement in the second half of Leinster's 21-18 European Champions Cup quarter-final victory over Ulster on Saturday.

"Leinster Rugby can confirm that Dan Leavy had an initial scan yesterday (Sunday) on a complex knee ligament injury but needs further assessment this week," Leinster tweeted on Monday.

"He has been ruled out for the remainder of the season and into next season to include the Rugby World Cup."

Leavy has endured a miserable season in contrast to last year when he played a key role in Ireland's Six Nations Grand Slam success and Leinster's Champions Cup triumph.

Having started the season well, he missed November's historic home win over world champions New Zealand and a nagging calf injury prevented him playing in Ireland's disappointing defence of their Six Nations title.