Donald Trump’s summit with Vladimir Putin in Helsinki last month was a “significant turning point” in Michael Cohen’s decision to turn on him, according to the attorney representing the US president’s former personal lawyer.

Lanny Davis claimed Cohen was “worried about the future of our country” after seeing the American leader siding with the Kremlin over his own intelligence agencies on the issue of Russian interference in the 2016 presidential election.

Mr Trump’s former “fixer” was one of his closest allies for more than a decade and once said he would “take a bullet” for him.

But he thrust the president into legal jeopardy this week after pleading guilty to eight criminal charges, including election campaign finance violations he alleged had been ordered by Mr Trump.

Mr Davis has insisted his client would refuse any offer of a presidential pardon from “a man that he considers to be a corrupt and a dangerous person”.

Cohen turned against Mr Trump after the president’s conduct “shook him up”, Mr Davis told MSNBC.

“Helsinki was a significant turning point, as he worried about the future of our country with the president of the United States aligning with somebody who everybody in his intelligence community… said interfered and tried to help Trump get elected,” he said. “Trump is the only one left denying that. And that shook up Mr Cohen.”

During a press conference with Mr Putin in Helsinki, Mr Trump said he did not “see any reason” to disbelieve the Russian president’s denial of election meddling, comments which put him at odds with US intelligence chiefs and were branded “treasonous” by political opponents in Washington.

Mr Trump later claimed he “misspoke” but has continued to denounce special counsel Robert Mueller’s investigation into Russian election interference as a “witch hunt”.

Cohen was left “seriously worrying about his unsuitability as president”, his lawyer said, adding: “It was an evolutionary process, a painful process.”

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Mr Davis has said his client “has information” that would be of interest to Mr Mueller and “wants to tell the truth” if questioned.

“Michael Cohen knows information that would be of interest to the special counsel, in my opinion, regarding both knowledge about a conspiracy to corrupt American democracy by the Russians and Russians and the failure to report that knowledge to the FBI,” the lawyer said.

Mr Trump is also facing allegations he orchestrated a cover-up to buy the silence of two women who claimed he had affairs with them.

Cohen has admitted paying hush money to adult film actress Stormy Daniels and former Playboy model Karen McDougal, and said he had violated election campaign finance laws “in coordination and at the direction of a candidate for federal office”.

Under mounting pressure, Mr Trump took to Twitter on Wednesday to attack his former lawyer, who he claimed was making up “stories in order to get a ‘deal”’ from federal prosecutors.

In an interview with his favoured news programme Fox & Friends, due to air on Thursday, the president argued the hush-money payouts were not “even a campaign violation” because he subsequently reimbursed Mr Cohen with his personal funds.

“In fact, my first question when I heard about it was did they come out of the campaign because that could be a little dicey,” said Mr Trump, who insisted he only found out about the payments after they were made by Cohen.

The president’s former lawyer has alleged under oath that Mr Trump instructed him to arrange the payments.

Mr Trump has previously denied knowing anything about a $130,000 (£101,000) payment to Stormy Daniels, who says she had sex with the billionaire Republican in a hotel room in 2006.

He later admitted knowing about the money, but said a non-disclosure agreement had been “used to stop the false and extortionist accusations made by her about an affair.”

In July, Cohen released audio tapes in which Mr Trump can be heard allegedly discussing making a payment to Daniels in September 2016.

Cohen's decision to plead guilty to campaign finance crimes is thought to have caught the White House off-guard, coming just an hour after the president's former campaign manager Paul Manafort was convicted of eight charges of bank and tax fraud.