Three senior past and present Foreign Office ministers, including the foreign secretary, Boris Johnson, were targeted by individuals identified by the FBI last week as central to their investigation into Trump-Russia collusion, the Observer has learned.

Tom Watson, deputy leader of the Labour party, called the revelations “extraordinary” and said the government must say whether other ministers were targeted or had meetings. The reports from the US had shocked MPs, he said, and it was vital to know if the Russian state had also sought to influence British politics.

The Observer has learned of meetings and encounters between British ministers and two individuals named in FBI indictments unsealed last week – George Papadopoulos, a foreign policy adviser for Donald Trump’s campaign, and a “London professor” with high-level connections to the Russian state, subsequently identified as a Maltese academic, Joseph Mifsud.

Alok Sharma, a Foreign Office minister until June this year and MP for Reading West, confirmed he had met Mifsud “a couple of times” and he had attended a fundraising dinner in his constituency on 19 October this year, where he had “briefly greeted” him.

An email from Mifsud to a colleague, seen by the Observer and uncovered by Byline, the crowdfunded independent journalism site, revealed Mifsud had told a colleague he would be “meeting Boris Johnson for dinner re Brexit” on that date. Sharma confirmed Johnson was the guest speaker at the event.

A Foreign Office source said: “The foreign secretary has not knowingly met this person, planned to meet this person, or indeed ever heard of this person before.”

Sharma said: “I did not introduce him to Boris Johnson and I don’t think anyone else did either.” But indications of Russian efforts to make contacts with British officials could prove embarrassing for Johnson, who was asked about possible foreign interference in Britain last week and replied: “I haven’t seen a sausage.”

The revelation comes as the Observer investigation into foreign influence places him in a web of relationships between a known Russian spy, Sergey Nalobin, expelled from Britain in 2015, and Matthew Elliott, the chief executive of Vote Leave, the official Leave campaign headed by Johnson.

Watson said: “We’re starting to have a much clearer picture from America of how the Russian state sought to influence the US election and I think there are multiple questions to be asked about how and in what ways the Russian state may have been exerting influence in British politics. Given the gravity of the allegations against Mr Papadopoulos, the government should make public any meetings these two individuals had with British officials and what was discussed.”

Even more questions are raised by a meeting the Observer has discovered between Papadopoulos and Tobias Ellwood, then a senior minister in the Foreign Office, at the UN general assembly in September 2016.

This was when Papadopoulos was still working for the Trump campaign and, according to the FBI’s documents, had made multiple contacts through his intermediary – the “London professor” – with “high-level Russian officials”. Ellwood’s meeting occurred after Papadopoulos had discovered in April that the Russians had “dirt on Hillary [Clinton]” in the form of “thousands of emails” but before WikiLeaks started publishing her emails in October.

Ben Bradshaw, the MP who has been one of the few voices asking questions about possible Russian interference in British democracy, said the Foreign Office’s explanation that such a meeting was “routine” was implausible. “In my experience, it is not normal for a minister to meet party campaign operatives while on official government business.” He added: “If Mr Papadopoulos’s role was as junior as Trump has been claiming, I would be surprised that a minister as senior and experienced as Mr Ellwood would agree to meet him.”

Tom Brake, the Liberal Democrat spokesman for Brexit, said it was time to launch a formal inquiry. “With concerns emerging about possible Russian interference here in the EU referendum, the [Commons] intelligence and security committee needs to be reconstituted as a matter of urgency.”