The Norwegian federation held their hands up to the mistake after the ruling by the Court of Arbitration for Sport (CAS).
In a statement the federation explained: "CAS has ruled that Martin Johnsrud Sundby violated the (World Anti-Doping Agency) rules for illegal use of an anti-asthma medication, Ventoline.
"The Norwegian ski federation accepts full responsibility in this affair because the federation thought that it wasn't necessary to ask for a medical dispensation to take this medication."
The federation's doctor Knut Gabrielsen told a press conference that 31-year-old Sundby had suffered from asthma since childhood.
Traces of the banned substance were detected in an anti-doping control Sundby took at Davos in December 2014 and at Toblach in January 2015.
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CAS accepted he had not taken the drug as a performance enhancer.
But for breaking the rules he was handed a two month ban, starting on July 11 and deprived of wins in the Tour de Ski and World Cup in the 2014/2015 season.