A top Senate Republican has released an FBI document detailing an allegation President Joe Biden and his son accepted bribes from a Ukrainian firm.

Chuck Grassley, who sits on the Senate Judiciary Committee, said: “The American people can now read this document for themselves.”

The unclassified document is a June 2020 internal FBI report outlining claims by a foreign informant.

There is no evidence that Joe Biden received any payments from Ukraine.

Senior congressional Republicans have acknowledged the allegation is unverified.

The memo contains an allegation that Hunter Biden was hired by the Ukrainian energy company, Burisma, in 2013 as a way to gain support from his father, who was the US vice-president at the time.

It also alleges that Burisma made two $5m (£3.9m) payments to “the Bidens” as part of its efforts to remove Ukraine’s prosecutor general at the time, Viktor Shokin – and that there were audio recordings of both Hunter and Joe Biden that supported this.

The informant told the FBI he had learned these details during conversations he had with the Burisma chief executive and other company officials in 2015 or 2016.

Vice-President Biden had publicly pressed for Mr Shokin’s removal as part of a larger effort by the Obama administration and US allies due to concerns that he was ineffective in tackling corruption in Ukraine.

Mr Grassley, an Iowa senator, says he obtained a lightly redacted copy of the memo from a justice department “whistleblower” and decided releasing it was in the public interest.

He and other Republicans have said the FBI considers the informant to be highly credible, but the report details how the informant said he could not vouch for the accuracy of the information he was passing along.

Mr Grassley tweeted a copy of the memo on Thursday. He said voters could now make their own minds up “without the filter of politicians or bureaucrats”.

Its public release, however, is a highly unusual move and could further inflame tensions between congressional Republicans and the FBI and Biden administration officials.

Since regaining a majority last year in the House of Representatives after five years of Democratic control, Republicans have launched a range of investigations into President Biden and his family.

The allegations detailed in the FBI document are similar to ones aired over the past several years by Mr Biden’s political opponents, including former Trump lawyer Rudy Giuliani, who had repeatedly travelled to Ukraine to conduct his own personal investigations.

Mr Giuliani’s accusations against the Bidens became a focus of Mr Trump’s first impeachment trial in 2019, which centred on the then-president’s efforts to pressure Ukrainian President Volodymyr Zelensky to announce he was launching an investigation into Joe Biden’s supposed ties to Burisma.

Bill Barr, the attorney general under Mr Trump, opened a justice department inquiry into the allegations in 2020.

He told the Federalist last month that he passed the case on to the US attorney in Joe Biden’s home state of Delaware.

No charges were ever filed in the case.

White House spokesman Ian Sams said: “It is remarkable that congressional Republicans, in their eagerness to go after President Biden regardless of the truth, continue to push claims that have been debunked for years.

“It’s clear that congressional Republicans are dead-set on playing shameless, dishonest politics and refuse to let truth get in the way.”

Congressional Republicans learned of the existence of the FBI document detailing the informant’s account earlier this year.

They had pressed FBI Director Chris Wray to provide it to the House Oversight Committee – going so far as to threaten to hold him in contempt of Congress if he refused to comply.

He eventually provided members of the committee with the opportunity to privately access a redacted version, but warned that a public release would jeopardise FBI intelligence-gathering methods.

“We have repeatedly explained to Congress, in correspondence and in briefings, how critical it is to keep this source information confidential,” the FBI said in a statement.

“Today’s release of the 1023 – at a minimum – unnecessarily risks the safety of a confidential source.”

After a private review of the document last month, Jamie Raskin, the ranking Democrat on the House Oversight Committee, said it contained only unfounded, second-hand accusations and no new details.

The FBI officials have described FD-1023 forms as a means to document “raw, unverified” intelligence information that does not reflect the conclusions of investigators.

“Recording this information does not validate it, establish its credibility, or weigh it against other information known or developed by the FBI in our investigations,” the agency said in a recent statement.

Even Senator Ron Johnson, a Wisconsin Republican who with Mr Grassley wrote a Senate report on Hunter Biden, has sounded sceptical about the memo.

According to the Cedar Rapids Gazette, he told a conservative radio show host Vicki McKenna last month: “This could be coming from a very corrupt oligarch who could be making this stuff up.”