Police and ambulance staff have rushed to a Mayfair street where a man died after a cardiac arrest.
Medics from the London Ambulance Service, who were called out to Albemarle Street in central London, near the Royal Institute at around 3.14pm, treated the man but he died at the scene.
The Metropolitan Police were called amid fears that he may have been poisoned.
A cordon has been thrown around the area.
A Scotland Yard spokesman said: "Due to concerns the victim may have been exposed to a poison, specialist officers attended but found no evidence that would indicate the victim had been poisoned.
"There are no other reports of people being taken ill.
"Cordons were put in place around the area to allow the emergency services to carry out their work."
A London Ambulance spokesman said: "We were called at 3.14pm today to Albemarle Street, Mayfair, to reports of a person in cardiac arrest.
"We sent a number of resources to the scene with the first of our medics arriving within five minutes.
"Sadly despite our efforts a man died at the scene."