Theresa May has said calls for an inquiry into alleged links between Russia and the campaign to leave the European Union are a matter for the police.
The Leave.EU founder Arron Banks has insisted there is "no evidence" the campaign "took Russian money" following reports he had more extensive contact with the Kremlin than previously acknowledged.
Millionaire Brexit backer Mr Banks and Leave.EU communications chief Andy Wigmore have promised "fireworks" when they are grilled by MPs on Tuesday.
They are due to give evidence before the Digital, Culture, Media and Sport Committee "fake news" inquiry.
Labour former Cabinet minister Ben Bradshaw asked the Prime Minister whether it was time for a "full police inquiry" into the relationship between Russia and the Leave campaign as she made a statement to the Commons on the G7 summit on Monday.
Mrs May replied: "As [Mr Bradshaw] asked me about police inquiries, that of course is a matter for the police and of course the body responsible for looking at elections and the democratic process is the electoral commission."
According to the Sunday Times, Mr Banks - the Leave.EU founder who helped bankroll Nigel Farage's campaign - held a series of undisclosed meetings with Russian embassy officials around the time of the 2016 referendum campaign.
The paper said it had seen emails showing he also discussed a potential business deal involving six Russian gold mines with ambassador Alexander Yakovenko after being introduced to him by a suspected Russian spy.
Addressing calls for a police inquiry on Nigel Farage's LBC radio show, Mr Banks said: "What's the crime? I would like to know what the police are going to investigate - lunch with the Russian ambassador?
He added: "Where's the evidence we took Russian money. There's no evidence."
Mr Banks said he first met the Russian ambassador over a "boozy lunch".
"We had about four shots of Stalin vodka, brandy, wine. We were pretty trollied. The ambassador was probably in a worse state than we were," he said.
He told Mr Farage there was another lunch meeting and a "cup of tea", where he was introduced to a businessman involved in the Russian gold mines, but denied using the Brexit campaign to further his business interests.
Asked about allegations he was reporting back to the Russians three days after meeting newly-elected Donald Trump, he said: "It's complete nonsense."
Mr Wigmore said one phone number was passed to the ambassador during the meeting so the Kremlim could contact Mr Trump's transition team.
Asked if there was any Russian money given to Leave.EU, Mr Wigmore insisted: "No, not one penny or one rouble."