Carina Trimingham, the partner of Liberal Democract Chris Huhne, has lost her High Court privacy and harassment action against Associated Newspapers.
Ms Trimingham, 44, has been ordered to pay £250,000 in costs within two weeks after losing her action over articles in the Mail and Mail on Sunday.
The PR adviser warned outside the High Court that the ruling could become a "blueprint for bullies and bigots" and said she would be appealing.
Ms Trimingham had sued for compensation and an injunction over 65 "highly unpleasant and hurtful" articles in the newspapers about her relationship with Mr Huhne.
She had an affair with the former energy secretary and senior Lib Dem, which culminated in June 2010 when the MP left his wife of 26 years to be with her.
Her lawyers told Mr Justice Tugendhat, sitting at London's High Court, that the articles had constituted a "cataclysmic interference" with her private life.
They referred to the "life and very different loves of the PR girl in Doc Martens" and described her as a "comedy lesbian from central casting".
Ms Trimingham's counsel, Matthew Ryder QC, had argued that the Mail had a right to freedom of expression but not to abuse his client repeatedly.
He told the High Court the articles had made constant and gratuitous references to Ms Trimingham's sexuality and her previous relationship with another woman.
But Associated Newspapers said in court that the stories were valid because there was an important public interest in Mr Huhne and the "after-shocks" of his marriage split.
Anthony White QC, for the publishers, said Ms Trimingham was "not a shrinking violet but a seasoned political journalist".
"She is open about her sexuality and, perhaps most telling, she has sold stories about other people's sex lives to the press. She gives as good as she gets, she dishes it out," he said.
In his ruling, Judge Tugendhat said: "Ms Trimingham was not the purely private figure she claims to be. Her reasonable expectation of privacy has become limited."
His decision will be considered a boost to freedom of expression and to newspapers fighting against tighter rules on privacy.