The story broke yesterday in the News of the World but the paper was unable to reveal the identity of the married woman with whom Blunkett, a divorced father of three, is having an affair.
The 43-year-old American has lived in the UK since 1987. She joined The Spectator in 1996 and is credited with helping the political magazine, edited by Boris Johnson MP, into profit for the first time.
Fortier joined from Conde Nast, where she had worked on the launch of GQ, and went on to to become the company's first communications director.
The Sun revealed Fortier had been involved with Blunkett since meeting him at a party in 2001.
It said "cheeky Kimberly", a brunette, introduced herself to Blunkett by jokily pretending she was tall and blonde.
The paper also said that despite working for the Tory magazine, Fortier describes herself as a "a nice Left-wing girl".
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