A major Brexit campaign donor held two previously undisclosed meetings with the Russian ambassador before and after the 2016 referendum and was offered a deal on six gold mines in Russia, The Sunday Times reported.
Arron Banks, the outspoken founder of the Leave.eu campaign, was introduced to London envoy Alexander Yakovenko by Alexander Udod, a "suspected Russian intelligence officer", the report said.
Banks has also reportedly admitted that he handed over phone numbers for members of Donald Trump's transition team to Russian officials, after meeting with the US president-elect in November 2016 in New York.
Asked about the report at the G7 summit in Canada on Saturday, Prime Minister Theresa May said: "I am sure that if there are any allegations that need investigation the proper authorities will do that."
Senior cabinet minister David Lidington told the BBC on Sunday that the allegations were "serious" and should be looked into by the authorities.
The paper said the revelations raised "explosive questions about attempts by Russia to influence the referendum result" but Banks dismissed them as part of a "witch-hunt" against Brexit and Trump.
- 'Family trip' to Moscow -
Banks, an insurance industry millionaire, has previously said he had "a six-hour boozy lunch" with Yakovenko at the Russian embassy on November 6, 2015.
Citing email correspondence, the Sunday Times said that meeting was set up by Udod and that he was one of 23 suspected Russian spies expelled by Britain following the poisoning of former double agent Sergei Skripal and his daughter Yulia in March.
The paper quoted Andy Wigmore, a close associate of Banks who was present at the meeting, as saying that they did not offer "any information to (the ambassador) or any Russian any details of our campaign".
The report said Banks and Wigmore met with Yakovenko again for a cup of tea on November 17 at a meeting that also included mining magnate Siman Povarenkin to discuss a deal involving six gold mines in Russia.
Banks, who is married to a Russian woman, then visited Russia in February 2016 at the height of the EU referendum campaign for what he said was "a family trip" that involved no meetings with Russian officials.
The paper said the Brexit campaigners subsequently invited Yakovenko and Udod to a drinks party in London's Notting Hill hosted by Banks, and the Russian ambassador was also asked to attend their results party in Westminster.
Banks and Wigmore were invited back to meet Yakovenko after they, and Brexit firebrand Nigel Farage, met with Trump in November 2016 shortly after his election.
"The ambassador was obviously keen to know how our meeting (with Trump) went," Wigmore said.
- 'Bite me' -
The Sunday Times report was based on emails passed to the paper by the journalist Isabel Oakeshott, who ghostwrote Banks's memoir "The Bad Boys of Brexit" and is now writing a book on Russian attempts to influence British politics.
The paper said she came forward after she said her email accounts were hacked.
"Banks and Wigmore were shamelessly used by the Russians," Oakeshott said in the Sunday Times.
But Banks told the paper: "I had two boozy lunches with the Russian ambassador and another cup of tea with him, bite me. It's a convenient political witch-hunt, both over Brexit and Trump."
He said nothing came of the discussion over the gold mine deal, saying: "We didn't profit from any business deals because I never pursued anything."
He also told the paper that he had disclosed details of his contacts with the Russians to US officials.
"We actually saw the suits from the American embassy who introduced us to the State Department to explain what had happened and then we briefed the Americans on our meetings with the Russians," he said.
Former FBI director Robert Mueller is currently investigating any possible collusion between Trump's 2016 election campaign team and Russia.
Trump has branded the investigation a "witch hunt".
British businessman Arron Banks founded the Leave.eu campaign