Three Bulgarians convicted of spying for Russia previously attended an event in the Palace of Westminster, a BBC News investigation has found.
Orlin Roussev, Biser Dzhambazov and Katrin Ivanova were present at an event to debate Brexit in a committee room in May 2016.
Photographs posted on social media and unearthed by the BBC show the spies with representatives of European political parties.
A parliamentary spokesperson said Parliament was a public building but that security processes were "robust".
Two people who knew Orlin Roussev during the relevant period confirmed he was the man pictured at the Commons event. The BBC visually reviewed the photographs to confirm Dzhambazov's attendance. Ivanova, also visible in photographs, appears on a list of people who indicated they would attend.
The use of the room for the event was sponsored by former West Ham MP Lyn Brown, now Baroness Brown of Silvertown. She was nominated for a life peerage by the Prime Minister Sir Keir Starmer.
Baroness Brown of Silvertown told the BBC she had "absolutely no memory" of the event and did not believe she had "met or spoken to the three individuals".
There is no suggestion of wrongdoing on the part of either the event's organisers or attendees.
