A police officer guarding former British prime minister Tony Blair left her loaded gun in a London coffee shop, where it was found by a member of the public.
The Sun newspaper's reported the female officer left the semi-automatic Glock 17 on the floor of the toilets in a Starbucks cafe.
It was found about 20 minutes later by an employee, who called the police.
London's Scotland Yard has confirmed the gun was left unattended.
The newspaper reports the plain-clothes policewoman in her 30s soon realised her mistake and spent nearly an hour looking for the weapon.
The gun was eventually traced back to her through its serial number.
An investigation has been launched into the incident.