Team Sky crossed an "ethical line" by using permitted medication to enhance the performance of Tour de France winner Bradley Wiggins, said a British parliamentary committee report published on Monday.
Team Sky immediately issued a statement to "strongly refute" the claim.
The long-awaited Digital, Culture, Media and Sport (DCMS) select committee report on 'Combatting doping in sport' said the widespread therapeutic use exemption (TUE) system was open to abuse.
It was unable to say for sure what was in a mysterious package delivered to Wiggins at the June 2011 Criterium du Dauphine race but found no "reliable evidence" to support Sky's assertion that it was a legal decongestant.
The committee said it believed a powerful corticosteroid, triamcinolone, was being used "to prepare Bradley Wiggins, and possibly other riders supporting him, for the Tour de France."
"The purpose of this was not to treat medical need, but to improve his power to weight ratio ahead of the race."
Wiggins, an asthma sufferer who has said he sought no unfair advantage, had been granted a TUE for the banned anti-inflammatory ahead of the 2012 Tour.
The report said that while there was no violation of the World Anti-Doping Agency (WADA) code, "it does cross the ethical line that (team principal) David Brailsford says he himself drew for Team Sky."
"In this case, and contrary to the testimony of David Brailsford in front of the Committee, we believe that drugs were being used by Team Sky, within the WADA rules, to enhance the performance of riders, and not just to treat medical need."
The report also accused Sky and British Cycling of "serious, unprofessional and inexcusable" behaviour in failing to keep proper medical records.
Team Sky said they took full responsibility for mistakes made in areas where they had already acknowledged they fell short.
"However, the Report also makes the serious claim that medication has been used by the Team to enhance performance. We strongly refute this," the statement said.
"The report also includes an allegation of widespread Triamcinolone use by Team Sky riders ahead of the 2012 Tour de France. Again, we strongly refute this allegation."
Sky said they were surprised and disappointed that the committee had "chosen to present an anonymous and potentially malicious claim in this way, without presenting any evidence or giving us an opportunity to respond."
The report also strongly criticised world athletics head Sebastian Coe for providing "misleading" answers to questions in a 2015 hearing about what he knew about doping in Russian athletics before he took office.
It said the reputation of both the Briton and IAAF had been affected.
"It stretches credibility to believe that he was not aware, at least in general terms, of the main allegations that the Ethics Commission had been asked to investigate," the report added.
Coe, a double Olympic 1,500 metres gold medallist, denied last year that he had misled the committee.
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