Lance Armstrong is being sued for £1m by The Sunday Times over the settlement of a libel action.
Armstrong, who has been stripped of the seven Tour de France titles he won between 1999 and 2005, received £300,000 from the newspaper in 2004 as payment towards his legal fees after an article written by David Walsh questioned the validity of his success after he had survived testicular cancer.
"It is clear that the proceedings were baseless and fraudulent," said a letter from lawyers representing The Sunday Times.
"Your representations that you had never taken performance enhancing drugs were deliberately false."
The paper, owned by Rupert Murdoch's News Corp., said its total claim against Armstrong is "likely to exceed" £1m.
"The Sunday Times is now demanding a return of the settlement payment plus interest, as well as its costs in defending the case," the paper said.
The newspaper is demanding the return of the £300,000 payment plus interest, as well as costs accrued in defending the case, which was eventually settled in 2006.
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Armstrong himself said the article was "based on untrue allegations, which are without substance".
Walsh told Huffington Post UK Sport Lord Justice Eady's judgement gave journalists an "excuse" not to unmask the truth.
His story - which was heavily diluted from his original copy - contained doping allegations made by Armstrong's former soigneur Emma O'Reilly.
A report by USADA in October said the now-retired Armstrong had been involved in the "most sophisticated, professionalised and successful doping programme the sport has ever seen"
Armstrong chose not to contest the USADA charges but has always denied using performance-enhancing drugs.