A leader of the Leave.EU campaign suggested sending a “message of support” to the Russian ambassador after the then foreign secretary made a speech that was critical of Russia, documents seen by the Observer suggest.

The material also appears to show that Andy Wigmore, spokesman for the Leave.EU campaign and the business partner of Arron Banks, the biggest funder of Brexit, passed confidential legal documents to high-ranking officials at the Russian embassy and then denied it to parliament.

The documents related to George Cottrell, an aide to Nigel Farage who was with him on the campaign trail for Donald Trump in July 2016. Cottrell was arrested by the FBI and charged with 21 counts of money laundering, bribery and wire fraud.

Damian Collins, chair of the culture, media and sport select committee, said that Banks and Wigmore appeared to have misled parliament and “what we really need to know is why”. He added: “It makes you question whose side they are on.”

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According to material seen by the Observer, Wigmore, who was Belize’s trade envoy to Britain at the time, forwarded an email to a Russian diplomat marked “Fw Cottrell docs – Eyes Only”. It is understood the email, dated 20 August 2016, showed six attachments of legal documents relating to Cottrell’s arrest by federal agents. It appears that Wigmore sent it to Sergey Fedichkin , a third secretary at the Russian embassy, saying: “Have fun with this.”

Collins asked Wigmore and Banks a series of questions about Cottrell’s arrest. He noted Wigmore was with Cottrell when federal agents seized him at Chicago airport on 26 July 2016, and that Farage was sent his charge sheet by the FBI. “Did you discuss George Cottrell’s arrest with the Russian embassy?” Collins asked. Wigmore replied: “It never came up. While at the time it probably seemed a big thing, there was so much else going on at the time it just was not an issue. It never came up.”

Collins told the Observer: “Wigmore kept trying to make the point that their contact with the Russian embassy was around social occasions, but we believe it went much further. On the surface, these documents didn’t hold any interest to the Russians, so why did they appear to pass them on? And why then deny it? Why did they mislead the committee about the true nature of their relationship? What are they trying to hide?”

The Observer has also seen what appears to be a discussion between the Leave.EU social media team and Wigmore and Banks in March 2016, three months before the referendum. On 11 March 2016 the Russian embassy put out a press release attacking Philip Hammond, the then foreign secretary, for suggesting that “the only country who would like us to leave the EU is Russia”.

Ian Lucas, Labour MP for Wrexham, who is also on the committee, said: “There has been a coordinated attempt to attack, bully and intimidate anyone asking questions about this, including MPs. But what the evidence is showing is an intimate business relationship with a hostile foreign government that was being built up in the period before the summer of 2016 that needs to be in the public domain.”

The Observer has seen a series of exchanges that suggest a picture of communications between the embassy and the Leave.EU campaign running up to the referendum which continued in the period after Farage became an active supporter and campaigner for Trump.

In October the Russian ambassador, Alexander Yakovenko, was identified by US special counsel Robert Mueller as a high-level intermediary between the Trump campaign and the Kremlin. The documents about Cottrell’s arrest appear to have been handed over during a period in which Trump’s campaign manager, Paul Manafort, had his business dealings in Ukraine exposed and was replaced by Steve Bannon.

The New York Times published a story about a secret ledger of payments to Manafort – that were paid via the British Virgin Islands, Belize and the Seychelles – on 14 August 2016. On 19 August, the day that Bannon became campaign manager, Wigmore and Banks were invited to lunch at the embassy with Yakovenko. And on 20 August, documents suggest, Wigmore appears to have sent the papers about Cottrell’s arrest.

A few days later Farage, along with Wigmore and Banks, travelled to meet Trump in Mississippi, where he introduced the crowd to “Mr Brexit” and promised to deliver “Brexit plus”.

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The indictment included claims about Cottrell’s expertise with the dark web and cryptocurrencies, and was public by the time Wigmore appears to have sent it to the embassy. It is also thought to include five other documents about the case.

The embassy said it “has not in any way intervened in the domestic UK political process, including the Brexit referendum.”