Democrats will open a formal impeachment inquiry into President Donald Trump over claims that he sought political help from Ukraine to investigate political rival Joe Biden, it was confirmed tonight.

Speaker of the House Nancy Pelosi, who has resisted calls to start the impeachment process for months, said President Trump’s actions represented a “betrayal of his oath of office, a betrayal of our national security and betrayal of the integrity of our elections.”

“The president must be held accountable. No one is above the law,” the top Democrat said.

“The actions taken to date by the president have seriously violated the Constitution, especially when the president says ‘I can do whatever I want.’”

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

The Herald: Speaker Nancy Pelosi has long resisted calls to start impeachment processSpeaker Nancy Pelosi has long resisted calls to start impeachment process

The President, who stepped away from a United Nations summit in New York and retreated to Trump Tower for some “executive time” ahead of Ms Pelosi’s press conference, hit back on immediately, slamming the announcement as “more witch hunt garbage”.

 “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!” Mr Trump tweeted.

The controversy erupted after it emerged US intelligence officials had complained to a government watchdog about Mr Trump’s interactions with a foreign leader, who was later revealed to be the president of Ukraine, Volodymyr Zelensky.

That whistleblower’s complaint – which was deemed “urgent” and credible by the intelligence inspector general – has been demanded by Democrats in Congress, but the White House and Department of Justice have refused to provide it.

It also emerged that Mr Trump ordered his staff to freeze nearly $400million (£320 million) in aid to Ukraine a few days before a phone call in which he reportedly pressured the eastern European nation’s leader to investigate the family of Joe Biden, who is frontrunner to take on the president in the 2020 elections. 

The President insists he was not withholding aid to Ukraine in the hopes the country would open an investigation into the former vice-president and his son, Hunter.

In remarks to reporters at the United Nations yesterday, he confirmed he held up the aid but said he did so to fight corruption and urge European nations to share in helping Ukraine.

“But very importantly, Germany, France, other countries should put up money and that’s been my complaint from the beginning,” he said.

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And, despite denying impropriety, Mr Trump acknowledged discussing political rival Joe Biden with the Ukrainian president. 

During meetings at the UN today, Mr Trump tweeted that an “unredacted” transcript of a call to Mr Zelensky will be released tomorrow. “You will see it was a very friendly and totally appropriate call. No pressure,” he tweeted.

Earlier today Joe Biden said that impeachment proceedings should begin unless the US president complies with investigations into his conversation with Ukraine’s leader.

Biden criticised the “baseless” corruption claims Trump has levelled against him, but he emphasised the true danger in the president’s actions were to the nation as a whole. 

“I can take the political attacks. They’ll come and they’ll go, and in time they’ll soon be forgotten,” the former vice president said. “But if we allow a president to get away with shredding the United States constitution, that will last forever.”