He claimed, for example, he was not aware at the time of his negotiations with the Ukrainians that Burisma, an energy company that Giuliani specifically named, had employed Hunter Biden, the former vice-president’s son.

Sondland had been first been due to give testimony on October 8 but was blocked from appearing by the Trump administration.

After the House committees issued a subpoena, Sondland defied state department orders, saying he was keen to testify.

The ambassador to the EU testified that Trump personally directed the drive to make the investigation a condition for a phone call from Trump to Volodymyr Zelensky, and a White House visit for the new Ukrainian president.

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It is a significant point in the impeachment proceedings being held by three House committees, which are looking for evidence Trump abused his office for political gain.

The key event in Sondland’s narrative is a May 23 White House meeting with Trump attended by Sondland, former Ukraine special envoy Kurt Volker and the energy secretary, Rick Perry.

The three had just returned from Zelensky’s inauguration and were trying to persuade Trump to have a telephone conversation with his new Ukrainian counterpart and invite him to the White House, as a mark of US support.

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“However, President Trump was sceptical that Ukraine was serious about reforms and anti-corruption, and he directed those of us present at the meeting to talk to Mr Giuliani, his personal lawyer, about his concerns,” Sondland said. “It was apparent to all of us that the key to changing the president’s mind on Ukraine was Mr Giuliani.

“Indeed, Secretary Perry, Ambassador Volker and I were disappointed by our May 23, 2019 White House debriefing,” Sondland said. “We strongly believed that a call and White House meeting between Presidents Trump and Zelensky was important and that these should be scheduled promptly and without any preconditions.

“We were also disappointed by the president’s direction that we involve Mr Giuliani,” Sondland added.

“Our view was that the men and women of the state department, not the president’s personal lawyer, should take responsibility for all aspects of US foreign policy towards Ukraine,” he said.

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“However, based on the president’s direction, we were faced with a choice,” Sondland argued. The choice was between abandoning their efforts to strengthen US-Ukrainian ties, “or we could do as President Trump directed”.

But the ambassador insisted: “I did not understand, until much later, that Mr Giuliani’s agenda might have also included an effort to prompt the Ukrainians to investigate Vice-President Biden or his son or to involve Ukrainians, directly or indirectly, in the president’s 2020 re-election campaign.”

When the diplomats talked to Giuliani, the lawyer said “the two anticorruption investigatory topics of importance for the president” was the allegation that a Democratic National Committee internet server had been located in Ukraine during the 2016 election (a far-right conspiracy theory) and Burisma’s activities.

Sondland added: “[M]y understanding was that the president directed Mr Giuliani’s participation, that Mr Giuliani was expressing the concerns of the president.”

He also claimed he was not aware at the time that US$400 million in US military aid had been withheld from Ukraine to add to the pressure on Zelensky.

Sondland stated such actions would be wrong but insisted he did not participate in them.