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The IRB appealed the findings of the independent judicial inquiry that found Thomson guilty of standing on the head of Scotland flanker Alasdair Strokosch at Murrayfield on November 11. Thomson was handed a two-week suspension that was reduced to one for his previous clean record and lack of intent.
The IRB claimed the sentence was too lenient - but lost the appeal, with the committee finding the initial judgement of a two-week suspension to be in line with the recommendations laid down by the governing body.
The committee agreed the act had been entry level, but they upheld the IRB's contention that the initial sentence should not have been cut in half, therefore meaning Thomson's ban was extended to two weeks.
The All Blacks had withdrawn him for selection for both the tests against Italy and Wales meaning the 30-year-old is able, if required, to play against England.
The outcome will be of some relief to Thomson who is still undecided about what he is doing in 2013. He did not want the test against Scotland - where he was yellow-carded at the time - to be his last.
The IRB, in a statement, welcomed the decision "to largely uphold the IRB's appeal against the one-week sanction handed down to Adam Thomson for trampling on an opponent's head.
"The IRB appealed the suspension believing it to be unduly lenient for the particular act of foul play. The IRB has a clear responsibility to ensure consistency of the application of the disciplinary process as set out in regulation 17 to protect the welfare of players at all levels."
- By Gregor Paul in Cardiff