Sam Warburton stayed in Melbourne, after Saturday's defeat against the Wallabies, to undergo a scan on his leg.
The Lions have now confirmed that he suffered a 'significant tear in his left hamstring.'
He will remain with the tour party, but will not play in the final test.
Warburton said: "It is incredibly disappointing to be ruled out of the tour through injury as we head to Sydney for the deciding match.
"I wish the team all the best and hope I can play some part in the build up this week. I am confident that the boys can finish the job off and secure the series win."
More top news
Winds will strengthen overnight as a band of blustery showers moves in from the west.
People from Wales have been describing what they saw following the Barcelona terror attack.
Cardiff City have announced the signing of 25-year-old Omar Bogle from Wigan Athletic on an undisclosed fee.