The Spanish cycling federation in October cleared Mayo, who tested positive for blood-booster EPO during the 2007 Tour de France, claiming his second sample was negative.
However cycling's world governing body the UCI said at the time the second sample still needed to be analysed.
The Spanish Federation have been given untill December 31 to look into the case again by the UCI.
Tour de France
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Should the RFEC decide not to punish the rider, who in theory would be banned for two years for a failed test, the UCI could then pass the case on to the Court of Arbitration for Sport in Lausanne.
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