Police investigating the death of oil executive Carole Waugh are questioning a woman on suspicion of conspiracy to defraud.
Miss Waugh, originally from Durham but living in London, was found with a single stab wound inside a car at a garage in Lime Court, New Malden, south-west London, on August 2.
Scotland Yard said the 61-year-old woman arrested on Friday is in custody at a south London police station. Ms Waugh, 50, was reported missing by her family on May 7. They last saw her at a family party in mid-April.
She had lived and worked in Libya for a number of years, but returned to London in 2008 and would regularly visit relatives in Durham and Cumbria. Her former boss, Stuart Anderson, told journalists she fled Libya after being threatened by someone in Muammar Gaddafi's regime, but a Scotland Yard spokesman said inquiries are focused in London.
Detectives said she met men through a variety of internet sites, including dating websites, and they were investigating the possibility that she might have been an escort.
So far, 11 people have been arrested in connection with the inquiry into Miss Waugh's disappearance. At least three women have tried to pose as her since she vanished, police said.
Her flat is in an affluent part of central London and her brother Chris previously said she was "very frugal and careful" with money.
Two men have been arrested on suspicion of murder. Rakesh Bhayani, 40, of Chamberlayne Avenue, Wembley, was held on Thursday and is still being questioned. He also faces three charges of conspiracy to commit fraud by false representation worth about £280,000, following transactions associated with Ms Waugh's identity and is due in court next month in connection with those.
Nicholas David Kutner, 47, of no fixed address, was arrested on suspicion of murder and kidnap last week. He also faces two charges relating to renting her property at Tressard Court and a fourth of unlawfully withdrawing funds from Ms Waugh's bank account.
Kutner is also facing a fraud charge after being accused of pretending to be Chris Waugh in a bid to sell her London home. He is in custody ahead of an appearance at Southwark Crown Court on Monday connected to that charge. Scotland Yard has appealed for information about the blue Volkswagen Golf Ms Waugh was found in, which has the registration number W466 NHL.