Donald Trump’s own words will take centre stage at the impeachment inquiry on Wednesday when his ambassador to the European Union faces questions about a phone call with the US president in a Ukrainian restaurant.

Gordon Sondland is the witness who most alarms officials at the White House, according to US media reports, fuelling speculation that the ambassador could plead the fifth amendment to protect himself from self-incrimination.

Along with the diplomat Kurt Volker and the energy secretary, Rick Perry, Sondland – a wealthy hotelier who donated $1m to Trump’s inaugural committee – was one of the so-called “three amigos” the Trump administration used to bypass normal state department channels to Ukraine.

He has already changed his closed-door testimony to admit he told an aide to the Ukrainian president, Volodymyr Zelenskiy, that military aid would not be released until Kyiv announced an investigation into a gas company linked to the son of the former vice-president Joe Biden, a potential challenger to Trump in next year’s election.

But even the revised statement failed to report that Sondland called Trump from a restaurant in Kyiv on 26 July and discussed “investigations”. The president was speaking loudly and the call was overheard by David Holmes, a political counsellor at the US embassy in Ukraine, according to testimony released this week.

Holmes told the House of Representatives’ inquiry: “I then heard President Trump ask, quote, ‘So he’s going to do the investigation?’ Ambassador Sondland replied that ‘he’s going to do it’, adding that President Zelenskiy will, quote, ‘do anything you ask him to’.”

Holmes said he had spoken to Sondland immediately after the call. In his opening statement, Holmes recalled: “Ambassador Sondland agreed that the President did not ‘give a shit about Ukraine’. I asked why not, and Ambassador Sondland stated that the president only cares about ‘big stuff’ … like the ‘Biden investigation’ that Mr [Rudy] Giuliani was pushing.”

The Trump call is not Sondland’s only headache. He is also likely to be cross-examined about a White House meeting with US and Ukrainian officials on 10 July at which he allegedly said the acting chief of staff, Mick Mulvaney, had made clear a meeting between Zelenskiy and Trump in Washington was conditioned on the investigations being launched.

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John Bolton, then the national security adviser, halted the meeting and said later he would not be “part of whatever drug deal Sondland and Mulvaney are cooking up on this”, the inquiry has heard.

Sondland, however, was reportedly undeterred and continued to pressure the Ukrainian officials in a separate room. The Daily Beast website reported: “Sondland continued to not just relay, but demanded ferociously, that the Ukrainians open the Biden investigations, saying it was the only chance for Washington and Kyiv to develop any further meaningful relationship, two individuals with knowledge of Sondland’s overtures said.”

Much of the ambassador’s evidence has been contradicted by other witnesses in the impeachment inquiry so far. On Tuesday, Tim Morrison, a former White House national security official, testified that Sondland said he had told a Ukrainian official that his government would have to announce investigations into Biden to free up the US military assistance.

But should Sondland buckle and reveal fresh details, he could face a fierce backlash from the president and his conservative allies. Trump tweeted criticism of Ambassador Marie Yovanovitch during her testimony and the White House did similar against Lt Col Alexander Vindman, who said Sondland referred to “specific investigations that Ukrainians would have to deliver in order to get these meetings”.