Sir Bradley Wiggins and Team Sky stand accused of crossing "an ethical line' by using permitted drugs to enhance the Tour de France winner's performance.
A British parliamentary committee says in a report published on Monday that the widespread therapeutic use exemption (TUE) system is open to abuse.
Wiggins, who suffers from asthma, took to Twitter to "strongly refute the claim that any drug was used without medical need".
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."
Team Sky also rejects the claims contained within the damning 'Combatting doping in sport' report.
The Digital, Culture, Media and Sport (DCMS) select committee says it can't say for sure what was in a mysterious package delivered to Wiggins at the June 2011 Critérium du Dauphiné 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."