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.

Katrin Ivanova was convicted of offences alongside Tihomir Ivanchev and Vanya Gaberova between August 2020 and February 2023, following a four-month trial at the Old Bailey in London.

Roussev, the UK spy cell's leader, and Dzhambazov, his primary lieutenant, had already pleaded guilty, along with another man, Ivan Stoyanov.

Together, they conducted surveillance operations across Europe targeting enemies of Vladimir Putin's regime, including investigative journalists Christo Grozev and Roman Dobrokhotov, as well as Russian dissidents and political figures.

The six, all based in the UK, will be sentenced this week.

A previous BBC investigation named and tracked down two women who were part of the group outside of the UK, who remain free.

The cell's activities were directed from Russia by Jan Marsalek, an Austrian former finance executive reported to be hiding in Moscow having fled fraud charges in Germany following the collapse of payments processing company Wirecard.

Marsalek is an asset of Russian intelligence services. A cache of tens of thousands of Telegram messages between Marsalek and Roussev recovered by UK police indicated close coordination with the Austrian's "friends in Russia".

Email exchanges seen by the BBC show Roussev was already in contact with Marsalek by the time of the May 2016 event in Parliament.

Roussev worked as the chief technology officer for a financial services firm after moving to the UK in 2009. The role also required him to spend time in Russia.

But the BBC has been told that he was sacked in 2012 after he was caught siphoning off significant amounts of money - $130k (£98k) - over the course of six months.

After being sacked, Roussev posed as a legitimate businessman at industry events.

The event in Parliament was organised and attended by representatives of European political parties, including the Bulgarian Socialist Party (BSP), which Dzhambazov and Ivanova joined in early 2016.

During the trial, messages between Dzhambazov and Roussev, in which the former boasted of political connections in the "upper echelon" of the BSP, were read into evidence. Since 2015, Dzhambazov had been actively involved in the administration of Bulgarian elections in the UK, via the Embassy. Ivanova and Gaberova also worked in this capacity at various junctures.

Photographs of the event in Parliament, published on Facebook, show Roussev, Dzhambazov and Ivanova seated behind Georgi Pirinski, a US-born former Communist who had served as chair of the Bulgarian National Assembly. At the time of the meeting he was a Member of the European Parliament for the BSP, which replaced the Bulgarian Communist Party in 1990.

Other attendees pictured include Roberto Sperenza, who served as Italy's Minister of Health under Prime Minister Guiseppe Conte between 2019 and 2022, representatives of the Social Democratic Party of Romania and a Labour Party councillor.

An entry on the BSP website published prior to the event claimed it would be attended by "representatives of all parliamentary parties in the UK parliament". It is not clear if that transpired.

Sir Iain Duncan-Smith MP, a former leader of the Conservative Party, said Parliament should investigate whether the group had accessed the building on any further occasions.

"Parliament should look into what was going on," he said.

"It's so easy to get in and out... Inside that building, they get a chance to bump into, meet, see and even go into offices if we're not careful."

A parliamentary spokesperson said: "Parliament is a public building, and we facilitate the visits of thousands of people to the estate each week.

"We have robust security processes in place, with the safety and security of all those who work in and visit Parliament our top priority.

"Members in both houses can sponsor events... in accordance with their own judgment," the spokesperson added.

Concerns have previously been raised about the security of Parliament in the face of alleged espionage activities by state actors.

In 2024, a former Parliamentary researcher, Christopher Cash, 30, denied spying for China. He will stand trial alongside Christopher Berry, 33, in October.

Two years earlier, MI5 warned an alleged Chinese agent, Christine Lee, had infiltrated Parliament to interfere in UK politics.