Sir Keir Starmer was given a personalised revolver with live ammunition as a gift by Turkish President Recep Tayyip Erdogan during the Nato summit.
The firearm, engraved with the prime minister’s name, was one of a series of similar gifts presented to the leader of each country who attended the event in Ankara.
Sir Keir’s revolver has not been brought back to the UK and remains with British officials in Turkey.
It is expected to be decommissioned before it is returned, meaning it will no longer be capable of firing live ammunition.
Erdogan waived export controls on the gift but it was left in Turkey because it is illegal to import a live firearm into the UK.
Downing Street has not released a picture of the revolver.
At the summit, Sir Keir signed a defence agreement with Erdogan which will pave the way for closer intelligence sharing between the countries.
Speaking after the summit, Sir Keir said he believed the alliance was stronger and more united following the gathering.
“This has been a good summit. We achieved what we wanted to achieve, which is unity.
“So important we have that, particularly with the conflicts going on in Ukraine and the conflicts in Iran,” he added.
It will be Sir Keir’s last major international event, after he announced his resignation last month.