Boris Johnson is facing questions about the government’s links to key individuals named by the FBI in its Trump-Russia investigation, following the emergence of a photo of him with Joseph Mifsud, the “London professor” with high-level Kremlin contacts.

The foreign secretary is facing accusations of a potential security breach following the emergence of the photo of him with Mifsud, whose identity emerged as part of investigations into alleged links between Donald Trump’s election campaign and Russia.

This development comes less than a week after Johnson denied meeting the professor, and at a time when concern is growing about possible Russian interference in the Brexit campaign, in which the foreign secretary played a crucial role.

Although the FBI had known about Mifsud’s role as a high-level go-between linking the Trump campaign and the Russian government since at least July, it appears British intelligence did not warn the foreign secretary about the potential embarrassment or security implications before he attended a fundraising dinner with Mifsud on 19 October.

Ben Bradshaw, the MP for Exeter, who has been raising questions about possible Russian interference, said: “It’s inconceivable that the FBI didn’t tell their UK counterparts about Mifsud … so how was this allowed to happen?” The only explanation, he suggests, is “our own agencies are keeping information from Johnson for some reason … which only begs further worrying questions”.

Last week, the Observer reported that Mifsud had told colleagues he was planning to meet Johnson “to discuss Brexit”. The third man in the photo, businessman Prasenjit Kumar Singh, told the Observer he had known Mifsud, a Maltese academic named in FBI documents concerning possible Trump-Kremlin connections, for several years, and had attended seminars on Brexit that Mifsud had held at the London School of Diplomacy: “He is a very good speaker and he’s had a lot of seminars about Brexit at the academy.”

Singh said they had met the foreign secretary at the event and had chatted to him “about normal things”. Asked if Mifsud had introduced himself to Johnson, he said: “I don’t know. I introduced myself to him and we talked. He is a very good man, a good politician.”

Last week, a spokesman said Johnson had “never met” Mifsud, a statement that was updated to “never knowingly met” after they confirmed his presence at the event. Last week a Foreign Office source said: “The foreign secretary has his photo taken with countless people he does not know, particularly during events attended by 150 people at which he’s the main speaker.”

Intriguingly, the photograph also presents new evidence of links to a further key individual named in the FBI documents.

On Friday, the New York Times identified a woman in the documents whom Mifsud introduced to a Trump campaign operative, George Papadopoulos, as “Putin’s niece”. The New York Times named her as Olga Polonskaya, 30, from St Petersburg, the former manager of a wine distribution company. The FBI documents state that she has high-level connections to the Russian government.

And she is also a contact of Singh’s, though he had no idea of her connection to the Trump-Russia investigation until told by the Observer. He said he had met her at Link Campus University in Rome, where Mifsud was based. “I had no idea she called herself ‘Putin’s niece’! She was just a normal student. Very nice. An ordinary girl.” He said she had rung him “about two and a half months ago” and had asked to meet.

“I was with my family and we were going to the Westfield shopping centre and I said: ‘Yes, come and meet me there.’ She was going to translate my website – for the London Executive School – from English into Russian so I could try and attract more Russian students. She did that: I just haven’t put it up yet.”

She had not been in touch since, Singh said. Nor had he heard from Mifsud who appears to have gone to ground. Polonskaya has now changed the privacy settings on her Facebook page, but a freelance investigative journalist, Gavin Sheridan, had already downloaded her data and discovered she had “liked” the photo of Joseph Mifsud and Boris Johnson and a status update of Prasenjit Kumar Singh that showed him with Theresa May.

Chris Bryant MP, the vice chair of the all-party parliamentary Russia group, said: “It’s all distinctly fishy. Boris Johnson’s relationship with the truth right now seems distinctly casual. We asked him about Russian interference in Brexit in the foreign affairs committee last week and he categorically denied he had seen a shred of evidence. I just thought ‘blimey’. Even as a junior minister in the foreign office, Russian stuff came across my desk every single day.”