Boris Johnson met an ex-KGB agent during a highly controversial trip to attend a party two days after attending a high-level Nato summit that focused on Russia, the Observer can reveal.

The prime minister, who was foreign secretary at the time, met Russian billionaire businessman Alexander Lebedev, whose family owns Britain’s Independent and Evening Standard newspapers, following a summit of foreign ministers in Brussels staged in the wake of the poisoning of ex-Russian agent Sergei Skripal.

The two met in Italy in April 2018, a month after the attack using the novichok nerve agent in Salisbury, when Johnson, in what appears to be a highly unusual break with protocol, apparently left behind his personal security detail and flew to a lavish party at a palazzo near Perugia hosted by Lebedev’s son Evgeny.

While the meeting with Evgeny Lebedev was confirmed to the Guardian in September, Alexander Lebedev denied meeting Johnson.

However a spokesman has gone on to acknowledge that the meeting between Johnson and Alexander Lebedev, a former KGB officer, did take place, though he insisted there was nothing was out of the ordinary.

“He goes there [Italy] often to stay with his son. He is in Europe a lot and has met pretty much all Evgeny’s friends here over the years. So there is nothing particularly unusual about the meeting,” said the spokesman for Alexander Lebedev.

It is not known what was discussed or how long the meeting between Moscow-based Alexander Lebedev and Johnson lasted. When asked to comment, Downing Street declined.

The 59-year-old Russian is a high-profile media mogul who has argued that western sanctions against Moscow over Ukraine have been ridiculous and ineffective, and whose stint in the KGB is well documented.

The development arrives amid ongoing disquiet over why Downing Street last month elected to suppress a secret intelligence report on the threats posed to UK democracy by illicit Russian activities in Britain.

Johnson had been expected to approve publication of the 50-page dossier, compiled by the cross-party intelligence and security committee, before parliament was dissolved on 5 November. Senior Whitehall sources close to the committee have told the Observer that the reason the dossier was shelved may have been Johnson’s relationship to Donald Trump and what the report says about the US president’s relationship to Russia.

A source also described Downing Street’s decision to block the report as “startling” and said that every reason that No 10 had subsequently put forward to explain the decision was “absolutely bogus”.

The source added: “It was a real shock. We hadn’t anticipated these sensitivities. It was a very carefully produced, non-partisan report. Nobody expected Downing Street to react in this way. There is something in this report that has caused alarm but I honestly don’t know what it is.”

On Friday Johnson insisted there was “no evidence” of Russian interference in the UK democratic process as pressure mounted for him to publish the report on the subject ahead of the general election.

Yesterday, shadow foreign secretary, Emily Thornberry, joined the calls for the findings to be made public. She said: “He should simply publish the report in full. If not, the British public will be left to draw their own conclusions.”

A spokesman for the Evening Standard and Independent who is also a spokesman for the Lebedevs told the Observer that Alexander Lebedev was an “outspoken critic of Putin”, that there was no suggestion he had “any relationship at all with the Kremlin”, and that “when the Lebedevs bought the papers, of course they would have been thoroughly vetted at that time”.

According to his Instagram account, Alexander Lebedev had been in Russian-occupied Crimea – he has been a vocal proponent of Russia’s occupation of the peninsula – a few days before travelling to Italy where he met Johnson.