A friendly foreign hand plunged America into a showdown over impeachment while President Donald Trump was practising international diplomacy with several leaders, including Prime Minister Narendra Modi, in New York.

House of Representatives Speaker Nancy Pelosi stunned the US by launching a formal impeachment inquiry into allegations that Trump had sought foreign help to smear Democratic rival Joe Biden ahead of next year’s election.

A few hours later, the White House released call records that suggested Trump had urged the President of Ukraine to contact attorney-general William P. Barr about opening a potential corruption investigation connected to Biden, a former Vice-President.

Speaker Pelosi’s decision to push forward with the most severe action that Congress can take against a sitting President could usher in a remarkable new chapter in American life. It can touch off a constitutional and political showdown with the potential to cleave an already divided America, reshape Trump’s presidency and the country’s politics, and carry heavy risks both for him and for the Democrats who have decided to weigh his removal.

The five-page “memorandum of telephone conversation” includes a cautionary note from the White House indicating that it is “not a verbatim transcript” but is based on “notes and recollections of Situation Room Duty officers” and national security staff. But senior administration officials said voice recognition software was used in preparing the document, which included long, direct quotations.

The memorandum reconstructs a July phone call that is at the centre of accusations.

“I would like you to do us a favour,” Trump said in response to President Volodymyr Zelensky of Ukraine’s raising of the prospect of acquiring military equipment from the US. Noting that the US had “done a lot for Ukraine”, Trump asked for another inquiry: that the Ukrainians examine an unsubstantiated theory about stolen Democratic emails.

“So whatever you can do with the attorney-general would be great,” the President told Zelensky, who later agreed to have the country’s new top prosecutor conduct the probes Trump wanted.

“The next prosecutor-general will be 100 per cent my person, my candidate,” Zelensky assured the President. “He or she will look into the situation.”

Trump’s suggestion that American law enforcement be directly involved and in contact with Ukraine’s government marks the first evidence that the President personally sought to harness the power of the US government to further a politically motivated investigation.

Trump has long pushed for Ukrainian officials to examine whether there was any improper overlap between Biden’s dealings with Ukraine while in office and his son’s position on the board of a Ukrainian energy company. There is no evidence so far to support Trump’s claim that Biden improperly intervened to help his son’s business in Ukraine.

Pelosi’s declaration of an impeachment inquiry, after months of reticence by Democrats who had feared the political consequences of impeaching a President many of them long ago concluded was unfit for office, is a stunning turn that has set the stage for a history-making and exceedingly bitter confrontation between the Democrat-led House and a defiant President.

Trump, who for months has dared Democrats to impeach him, issued a defiant response on Twitter with a series of fuming posts that culminated with a simple phrase: “PRESIDENTIAL HARASSMENT!”

Pelosi said: “The actions taken to date by the President have seriously violated the Constitution.” Trump, she added, “must be held accountable — no one is above the law”.

She said the President’s conduct revealed his “betrayal of his oath of office, betrayal of our national security and betrayal of the integrity of our elections”.

Although the outcome is uncertain, it has raised the possibility that Trump could become only the fourth President in American history to face impeachment.

Presidents Andrew Johnson and Bill Clinton were both impeached but later acquitted by the Senate. President Richard M. Nixon resigned in the face of a looming House impeachment vote.