Tests on the body of MI6 spy Gareth Williams have revealed no traces of any foreign substances that may have led to his death.
No evidence of drugs, alcohol or poisons were found during a battery of tests conducted by toxicologists, sources close to the inquiry said.
The 30-year-old GCHQ code-breaker's naked and decomposing body was found in a padlocked holdall in the bath of his Pimlico flat on August 23.
The mysterious circumstances of Mr Williams's death sparked an international frenzy of speculation earlier this year.
Coroner Dr Paul Knapman is due to review the case in private next Wednesday, after opening an inquest on September 1.
A Metropolitan Police spokesman said: "Results from comprehensive further toxicology tests carried out in relation to the death of Gareth Williams have come back negative, showing no trace of any drugs, alcohol, poisons or any other substances that would indicate cause of death.
"There are no plans to carry out any further tests of this type, but inquiries continue to try and establish a formal cause of death."
Scotland Yard detectives, led by Detective Chief Inspector Jackie Sebire, continue to try and identify what led to Mr Williams's death. They have been hunting for a man and a woman seen calling at the communal door of his Alderney Street home.
Witnesses said the Mediterranean couple, aged in their 20s, were let into the property late on an evening in June or July.
There was no sign of any forced entry to the property or a disturbance inside and nothing is believed to be missing. Mr Williams, of Anglesey, north Wales, was last seen alive on August 15, eight days before he was found dead in the £400,000 property.