Hunter Biden lobbied the US government for help in securing a lucrative energy contract in Italy while his father Joe Biden was vice-president, newly released documents show.

A tranche of previously undisclosed documents now made public under a Freedom of Information Act (Foia) request reveals that the president’s son wrote to the then US ambassador to Rome, John Phillips, in 2016 seeking assistance on behalf of the Ukrainian energy company, Burisma, of which he was a board member.

The request was revealed in documents issued by the US state department following a protracted effort by the New York Times seeking their release that lasted more than three years.

While there is no evidence that the government met Hunter’s request for help or that his father knew of it, the revelation is likely to fuel Republican claims that Joe Biden’s political position was used to help his son’s business activities.

The revelations come as Hunter Biden prepares to stand trial in California next month on charges of tax evasion on his income from Burisma and other foreign businesses.

They follow his conviction in June on charges of illegal gun ownership during a time when he was using crack cocaine.

Republicans have sought for years to tar Joe Biden with his son’s business activities, alleging that they reveal evidence of family corruption in a fruitless impeachment effort against the president last year.

Even before the latest release, James Comer, the Republican chair of the House oversight committee, which has spearheaded the congressional investigation into Hunter Biden’s business affairs, called the affair “the biggest political corruption scandal of our history’s lifetime”, in remarks last week to the rightwing channel, Newsmax.

The pursuit of Biden has lost much of its political sting since his withdrawal from the presidential race last month, after weeks of pressure from fellow Democrats following a disastrous debate performance.

The New York Times reported that the decision to release the documents containing the latest revelations was taken weeks before Biden stepped aside in favour of the vice-president, Kamala Harris, meaning that it was unlikely that the documents had been deliberately withheld until they had become less politically damaging. The White House signed off on the release a week before the president announced his withdrawal, the paper reported.

The state department has a track record of being slow to release records in response to requests. The Times said it had challenged the thoroughness of previous releases because they failed to include files stored in a laptop that Hunter Biden had left at a computer repair shop in Delaware.

Hunter’s letter to the ambassador – the contents of which were entirely redacted – appeared intended to help secure a meeting with the head of the regional government in Tuscany in furtherance of a geothermal energy project for which Burisma was seeking regulatory approval for in the Italian region.

It elicited a cautious response from US officials at the embassy and there is no evidence that a meeting ever took place as a result.

“I want to be careful about promising too much,” a US commerce department official based in the embassy wrote in response.

“This is a Ukrainian company and, purely to protect ourselves, U.S.G. [United States government] should not be actively advocating with the government of Italy without the company going through the D.O.C. [Department of Commerce] Advocacy Center.”

Enrico Rossi, the president of the Tuscan regional government at the time, told the New York Times that he never met Hunter Biden and had no recollection of the US embassy contacting about the Burisma project.

A White House spokesman said Joe Biden was not aware of his son’s letter to the ambassador at the time.

Hunter Biden’s lawyer, Abbe Lowell, called his client’s outreach to the ambassador a “proper request”.

“No meeting occurred, no project materialised, no request for anything in the US was ever sought and only an introduction in Italy was requested,” Lowell said in a statement.