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."
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More than half of adults in Wales have experienced harm or a negative experience because of someone else's drinking, a new report has found.
A major review will be published later this morning into student finance and higher education in Wales.
Dull and damp for much of Wales as we start the day, with drier and brighter weather on the way this afternoon.