Prentice Steffen, a 44-year-old sports medicine specialist, said he was sacked by US Postal in 1996, before Armstrong's arrival, when he refused to administer doping products to certain riders.

Armstrong, the now-retired seven-time Tour de France winner, has been fighting off allegations of doping since L'Equipe newspaper published the results of retroactive doping tests from the 1999 Tour de France.

The report said the tests, carried out only in 2004 because a test for the banned blood booster erythropoietin (EPO) did not then exist, showed Armstrong had tested positive six times during that year's race - which Armstrong has vehemently denied.

Last week, however, the International Cycling Union (UCI) was forced to finally act and appointed an independent investigator to look into the affair which has placed further question marks over Armstrong's integrity.

Steffen first aired his fears in 2001 when he told The Sunday Times newspaper that Tyler Hamilton and Marty Jemison - former members of US Postal - had approached him asking for help in improving their performances.

Hamilton, who since moved to the Swiss Phonak team, is currently awaiting a decision by the Court of Arbitration for Sport on his ban for blood doping following positive tests at last year's Tour of Spain.

Steffen, who claimed he had received threatening phone calls from Armstrong warning him not to speak out, said in an interveiw with L'Equipe last Friday that if Armstrong or Hamilton escape punishment then he would end his 26-year links with the sport.