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
There are calls for dog owners to do more to control their animals after a spate of attacks on dogs in Port Talbot.
Cloudy with rain continuing, mainly affecting western-facing hills.
The last surviving members of the north Wales branch of the Normany Veterans Association held their last formal meeting today.