Missing oil executive Carole Waugh, whose body was found in a lock-up garage, had suffered a single stab wound, police said.
Miss Waugh, 50, originally from Durham, had not been seen by her family since mid-April and concerns for her safety grew after a fraudster tried to sell her central London flat by posing as her brother.
Scotland Yard said a post-mortem examination held at Kingston mortuary found a single stab wound on the body. Further examinations are expected to establish the exact cause of Miss Waugh's death.
Miss Waugh's body was found in a car inside the lock-up garage in Lime Court, New Malden, south-west London, on Friday night and formally identified on Friday.
Police announced on Wednesday that Miss Waugh's disappearance was being treated as a murder inquiry and arrested a 47-year-old man on suspicion of kidnap, conspiracy to defraud and murder. He is in custody at a south London police station.
Suspicious financial transactions running to thousands of pounds have also been made using Miss Waugh's identity since she vanished, and her Cartier jewellery is missing.
Scotland Yard has appealed for information about the blue Volkswagen Golf where Miss Waugh was found, which has the registration number W466 NHL. Detective Chief Inspector John McFarlane said: "We need to hear from anyone with information on the car, particularly since April this year. Anyone who can assist us on where it has been since then, or can advise on anyone with a connection to it, should come forward."
Miss Waugh, who owned a flat in Harrowby Street, Marylebone, 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.
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, a total of 10 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.