A second whistleblower has come forward “with first-hand knowledge” of explosive allegations against Donald Trump that triggered an impeachment inquiry.

In what could be a damaging blow to the president’s repeated denials of wrongdoing over his phone call with Ukraine’s leader, the official is said to have already been interviewed by Michael Atkinson, the inspector general of the US Intelligence Community.

News of the development was revealed by the attorney representing the whistleblower who first reported Mr Trump’s phone call with Ukraine’s Volodymyr Zelensky, in which he pressured him to investigate key political rival and potential 2020 presidential candidate Joe Biden.

The attorney, Mark Zaid, told ABC News he was now representing both officials, adding that they are protected in law from being fired in retaliation.

He said the latest whistleblower had not yet spoken to congressional committees investigating the alleged offences.

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Mr Zaid said he did not know whether this second whistleblower is the same official that The New York Times claimed was considering whether to testify to congress about the Ukraine scandal.

However, he said that they worked in intelligence and had “first-hand knowledge that supported” the original whistleblower, although they have not yet filed a formal complaint with the inspector general.

In the coming week the impeachment inquiry is expected to intensify, with two key protagonists in the scandal testifying to congress.

On Tuesday the three House of Representatives committees leading the investigation will hear from Gordon Sondland, a hotelier who became US ambassador to the European Union after donating $1m to Mr Trump’s inauguration fund.

Text messages released last week suggest Mr Sondland was involved in efforts by Rudy Giuliani, the president’s private attorney, to dig up dirt on Mr Biden and link interference in the 2016 US election to Ukraine. In one text message, he appears to dismiss fears from the US ambassador to Ukraine, William Taylor, that withholding military aid to Kiev until politicians there committed to investigations of Mr Trump’s political enemies represented a quid pro quo.

Trump with Sondland in Brussels last year (AP)

On Friday congress is expected to hear testimony from Mr Taylor’s predecessor as US ambassador to Ukraine, Marie Yovanovitch.

A career diplomat, she was abruptly recalled from her posting in May this year. It has been suggested this was because she was refusing to cooperate with Mr Giuliani’s efforts.

Mr Trump appears to refer to her several times in the partial transcript of the July phone call with Mr Zelensky, which triggered the impeachment probe, referring to her as “the woman” and “bad news” and saying: “Well, she’s going to go through some things.”

Committee members will hope that both Mr Sondland and Ms Yovanovitch will be able to shed light on whether Mr Trump was effectively running a shadow foreign policy in Ukraine dedicated to smearing his enemies and chasing conspiracy theories about the 2016 election.

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Mr Trump began the day by lashing out again at a fellow Republican, the Utah senator and former presidential candidate Mitt Romney, who has cautiously criticised the president over the Ukraine scandal.

The president tweeted: “The Democrats are lucky that they don’t have any Mitt Romney types. They may be lousy politicians, with really bad policies (Open Borders, Sanctuary Cities etc.), but they stick together!”

Later he returned to his attacks on Mr Biden and his son, Hunter, repeating accusations of corruption in the latter’s business dealings in Ukraine and China, but again offering no evidence.

He wrote: “It is INCREDIBLE to watch and read the Fake News and how they pull out all stops to protect Sleepy Joe Biden and his thrown out of the Military son, Hunter, who was handed $100,000 a month (Plus,Plus) from a Ukrainian based company, even though he had no experience in energy,...

“....and separately got 1.5 Billion Dollars from China despite no experience and for no apparent reason. There is NO WAY these can be legitimate transactions? As lawyers & others have stated, as President, I have an OBLIGATION to look into possible, or probable, CORRUPTION!”