After months of having resisted calls from members of her party to press ahead with a censure of a man many believe has abused his position many times over, the House speaker said the president had crossed a line with his controversial call to the president of Ukraine.

“The president must be held accountable. No one is above the law,” she said on Capitol Hill.

“Therefore today I’m announcing the House of Representatives is moving forward with an official impeachment inquiry.”

In setting in motion formal impeachment hearings against a sitting president for just the fourth time in the nation’s history, Ms Pelosi is taking a considerable risk.

Everyone Trump has fired or forced out Show all 13 1 /13 Everyone Trump has fired or forced out Everyone Trump has fired or forced out John Bolton Trump claimed to have fired Bolton, his national security adviser, while Bolton claimed he offered to resign. An anonymous White House source that Bolton's departure came as a result of the national security adviser working too independently of the president AFP/Getty Everyone Trump has fired or forced out Anthony Scaramucci Scaramucci lasted only six days in his role as Trump's communications director before being fired by John Kelly, the incoming chief of staff Getty Everyone Trump has fired or forced out Rick Perry Rick Perry announced his resignation just as he became embroiled in the president's impeachment scandal. The White House said Mr Perry was asked by Donald Trump to work with Rudy GIuliani in regards to Ukraine. AP Everyone Trump has fired or forced out Rex Tillerson Tillerson, Trump's first secretary of state, was fired after a series of clashes with the president over policy Getty Everyone Trump has fired or forced out James Mattis Mattis served as secretary of defense from the beginning of Trump's administration until retiring on 1 January 2019. However, the president later claimed that he had "essentially fired" Mattis Getty Everyone Trump has fired or forced out James Comey Comey was fired as director of the FBI early in Trump's presidency after serving in the role for four years prior. His dismissal is widely thought to have been related to the Russia investigation Getty Everyone Trump has fired or forced out Reince Priebus Priebus, Trump's first chief of staff, was forced out after six tumultuous months AFP/Getty Everyone Trump has fired or forced out David Shulkin Veterans affairs secretary Shulkin claims that he was fired, the White House claims that he resigned Getty Everyone Trump has fired or forced out John Kelly Kelly, Trump's second chief of staff, was forced out after 17 months in office. His departure was a confused affair though it is clear that Trump wanted Kelly out AFP/Getty Everyone Trump has fired or forced out Michael Flynn Flynn lasted 24 days as Trump's national security adviser before being fired for lying to the FBI Getty Everyone Trump has fired or forced out Lee Cisna Cisna served as director of citizen and immigration services between October 2017 and June 2019 before being asked to resign amid a major personnel change in the department of homeland security Everyone Trump has fired or forced out Madeline Westerhout Westerhout served as Trump's personal assistant after leaking private information about his family AFP/Getty Everyone Trump has fired or forced out Mira Ricardel Ricardel was forced out of her role as Deputy National Security Advisor after first lady Melania Trump publicly called for her to be fired

She has long feared such a move could energise the president’s own supporters, ahead of an election campaign that is already on course to be ugly and bitter. She also knows that when Republicans in the House impeached Bill Clinton in 1998 – the move was not subsequently supported by the Senate – a number of senior Republicans suffered at the hands of voters when they next went to the polls.

The clearest indication of how Mr Trump’s supporters will react to the move, came from the president himself who immediately responded in a tweet storm while still in New York, where earlier in the day he addressed the United Nations General Assembly.

“Such an important day at the United Nations, so much work and so much success, and the Democrats purposely had to ruin and demean it with more breaking news Witch Hunt garbage. So bad for our country,” he wrote.

“They never even saw the transcript of the call. A total Witch Hunt!”

He added, in capital letters: “PRESIDENTIAL HARASSMENT!“

If Trump doesn't cooperate with Congress then there is 'no choice' but to start impeachment, Joe Biden says

An impeachment inquiry had already technically been underway since Jerry Nadler, chair of the House Judiciary Committee, last month filed a series of legal motions seeking specially protected evidence from special counsel Robert Mueller’s inquiry into Russia’s alleged interference in the 2016 election.

But the announcement by Ms Pelosi, who told members of her party that investigative work currently being carried out by six Democratic-controlled committees should operate under an impeachment umbrella, indicates the move now has her full support and is being carried out in the name of the most powerful woman in Washington.

Reports suggested the six various committees would examine the most egregious examples they can find of Mr Trump’s behaviour and the party would decide whether there was sufficient evidence to move forward to a vote, first by the Judiciary Committee, and then the full House.

The move by Ms Pelosi appeared to catch Mr Trump by surprise. On Tuesday morning at the UN, he was defending the contents of his July 25 conversation with Ukraine president Volodymyr Zelensky, in which he urged him to investigate Democratic presidential frontrunner Joe Biden and his son.

Mr Trump promised to release a transcript of his phone call. He also confirmed he had withheld nearly $400m in US aid to Ukraine but denied he did so as leverage to get Mr Zelensky to initiate an investigation that would damage the former vice president.

“We’re supporting a country. We want to make sure that country’s honest,” Mr Trump had told reporters on Monday. “It’s very important to talk about corruption. If you don’t talk about corruption – why would you give money to a country you think is corrupt?”

Mr Trump found himself in hot water earlier this month after it was revealed a whistleblower, believed to be a member of the US intelligence services, had raised the issue of the president’s call.

On Tuesday, House Intelligence Committee chairman Adam Schiff said the official who first made the complaint about the phonecall had made contact with Joseph Maguire, the acting Director of National Intelligence (DNI).

“We have been informed by the whistleblower’s counsel that their client would like to speak to our committee and has requested guidance from the acting DNI as to how to do so,” Mr Schiff tweeted.

He added: “We’re in touch with counsel and look forward to the whistleblower’s testimony as soon as this week.”