Nancy Pelosi has announced an official impeachment investigation into US President Donald Trump over an alleged abuse of power, saying his actions were a "betrayal of national security".
Pelosi, the Democratic speaker of the House of Representatives, said the President had "breached his constitutional responsibilities".
The decision comes amid reports that Trump may have pressured a foreign leader to investigate former vice president and potential 2020 campaign rival Joe Biden and his family.
Trump has lashed out at calls for an impeachment inquiry as a "witch hunt".
Impeachment is a rare and extraordinary step that would overturn the decision of US voters in 2016 to elect Trump.
The US Constitution permits Congress to remove presidents before their term is up if enough lawmakers vote to say that they committed "treason, bribery, or other high crimes and misdemeanours."
Only three presidents have been subjected to impeachment proceedings. Two were impeached — Andrew Johnson in 1868 and Bill Clinton in 1998 — but ultimately acquitted and completed their terms in office. A third, Richard M. Nixon, resigned in 1974 to avoid being impeached.
• The impeachment process, explained
Pelosi said: "Today, I'm announcing the House of Representatives is moving forward with an official impeachment inquiry.
"I'm directing our six committees to proceed with their investigations under that umbrella … The president must be held accountable.
"I can say with authority the Trump administration's actions undermine both our national security and our intelligence and our protections of whistleblowers."
Pelosi noted the political scandal over Trump's attempt to seek dirt from the Ukraine on his potential 2020 Democratic presidential rival Joe Biden, adding: "The actions taken to date by the president have seriously violated the constitution, especially when the president says, 'Article Two says I can do whatever I want'.
"The actions of the Trump presidency revealed dishonourable fact of the president's betrayal of his oath of office, betrayal of our national security, and betrayal of the integrity of our elections," she said.
"The President must be held accountable. No one is above the law."
The House Speaker has previously shown little appetite for impeachment during the first three years of Trump's tumultuous presidency, despite a push among Democrats in the US Congress to impeach Trump having gained momentum in that time.
The matter became the subject of a whistleblower's complaint which alleged Trump pressured Ukraine to investigate Biden and his son despite a lack of known evidence that either did anything wrong.
Pelosi's decision to begin an impeachment inquiry sets up an election season clash between Trump and Congress that seems certain to exacerbate the nation's fierce partisan divides and inject deep uncertainty into the 2020 presidential contest.
Pelosi has spent months trying to keep an impeachment inquiry at bay. But her position became untenable this week as more members — including crucial moderates in political swing districts — swung in favour of a probe following reports that Trump pushed Ukraine's leader for help investigating Democrat Joe Biden and his son during a summer phone call.
Pelosi's change of heart comes after days of consulting allies and follows reports that Trump may have pressured a foreign leader to investigate former vice president and potential 2020 campaign rival Joe Biden and his family.
Those reports over a week created a groundswell of support among Democrats for impeachment, with moderates from swing districts joining liberals in calling for an impeachment inquiry.
Pelosi said that those allegations forced her to reconsider her position that impeachment is too divisive for the country.
"The president is making lawlessness a virtue in our country," Pelosi said.
"We don't ask foreign governments to help us in our elections. That's what we tried to stop with Russia. It's wrong."
Trump has lashed attempts to impeach him, claiming he is the victim of the "Greatest and most Destructive Witch Hunt of all time".
The President said he had authorised the release of a transcript of the call on Wednesday.
"You will see it was a very friendly and totally appropriate call," Trump said.
How does impeachment work?
If politicians believe a president is guilty of what the US Constitution calls "treason, bribery, or other high crimes and misdemeanours," the process begins in the House of Representatives.
Any member can introduce an impeachment resolution, which like any other bill would be sent to a committee, most likely the House Judiciary Committee.
The committee can review the evidence it receives, or carry out an investigation itself.
If the evidence is strong enough, the committee crafts articles of impeachment — criminal charges — and sends them to the full House.
The House can pass the articles by a simple majority vote, "impeaching" the president.
The articles then go to the Senate, where a trial takes place, with representatives from the House acting as prosecutors and the president and his attorneys presenting his defence.
The chief justice of the Supreme Court presides over the trial in the Senate. The 100-member Senate then votes on the charges, with a two-thirds majority necessary to convict and remove the president.
If the president is convicted, the vice president would then take over the White House.
What kind of charges do presidents face?
The accusations have to meet the constitutional standard of "high crimes or misdemeanours," which is very broad.
In the cases of Clinton and Nixon, independent prosecutors conducted extensive investigations and amassed evidence to support criminal charges.
Nixon was accused of obstruction of justice, abuse of power, and contempt. Clinton, in the Monica Lewinsky scandal, was accused of perjury and obstruction. Trump could conceivably face charges of abuse of power for using his office to pressure Ukraine to conduct a politically-motivated investigation of Joe Biden and his son, Hunter, who had business dealings in Ukraine.
Special Counsel Robert Mueller, in the Russia election meddling investigation, also detailed multiple instances of alleged obstruction of justice by Trump that could arguably support charges.
Is it about law or politics?
Both. Given the momentous nature of an effort to remove the president, a clear crime with strong evidence — stronger than for an average citizen — is required.
At the same time, it is very much a political decision.
In past impeachment proceedings, support and opposition ran along party lines, though in Nixon's case the offences were so egregious that Republican backing for him quickly disintegrated.
In Democrat Clinton's case, Republicans controlled the entire Congress. But when impeachment charges went to the Senate, the 45 Democratic Senators stayed united to block a two-thirds vote for conviction.
With Trump, Democrats are divided for political reasons.
Pelosi has previously argued that impeaching Trump would go nowhere in the Republican-controlled Senate and could damage the party's effort to win full control of the Congress and the White House in the November 2020 elections.
Others in the party say Trump needs to be held accountable — that Democratic voters demand it.
Washington Post, news.com.au