The acting US ambassador to Kyiv has provided congressional committees conducting impeachment hearings a detailed account of how Donald Trump repeatedly sought to make a summit meeting and military aid to Ukraine conditional on its government launching investigations into the president’s political opponents.

Bill Taylor’s testimony was the latest in a series of depositions by serving and former administration officials, as part of the impeachment inquiry, about Trump’s use of the presidency to put pressure on the Ukrainian government to procure compromising information on his political rivals. And it was the most detailed and damning to date.

Taylor, who took over as acting ambassador in June, presented his testimony behind closed doors on Capitol Hill but copies of his opening statement soon leaked.

The veteran diplomat said that soon after arriving in Kyiv, he became concerned “our relationship with Ukraine was being fundamentally undermined by an irregular informal channel of US policymaking, and by the withholding of vital security assistance for domestic political reasons”.

Taylor said this irregular channel was run by Trump through several emissaries: the president’s personal lawyer, Rudy Giuliani, the departing energy secretary, Rick Perry, the ambassador to the European Union, Gordon Sondland, and the special Ukraine envoy, Kurt Volker.

They became focused solely on persuading the new Ukrainian president, Volodymyr Zelenskiy, to announce investigations that would damage Democrats and especially Joe Biden, the former vice-president and leading contender to the be the 2020 Democratic nominee.

Democrats declared it to be the clearest account to date of Trump’s abuse of office in the Ukraine scandal.

The White House spokeswoman, Stephanie Grisham, issued a statement denouncing the congressional hearings as “a coordinated smear campaign from far-left lawmakers and radical unelected bureaucrats waging war on the constitution”.

According to Taylor’s statement, published by the Lawfare website, Sondland, made clear in a phone call that both military aid and a White House meeting with Trump were dependent on the launch of two investigations.

One was into a Ukrainian energy company, Burisma, which had employed Hunter Biden, the son of former vice-president and 2020 Democratic contender Joe Biden. The second was into Ukraine’s role in the 2016 presidential election, a reference to a conspiracy theory that – counter to the consensus view of US intelligence agencies – held that it was Ukraine that had interfered in the vote in the Democrats’ favour, rather than Russia in favour of Trump.

“During that phone call, Ambassador Sondland told me that President Trump had told him that he wants President [Volodymyr] Zelenskiy to state publicly that Ukraine will investigate Burisma and alleged Ukrainian interference in the 2016 election,” Taylor said in his statement.

He added: “Sondland said ‘everything’ was dependent on such an announcement, including security assistance … President Trump wanted president Zielinski in a ‘public box’ by making a public statement about ordering such investigations.”

Although the first incentive for Zelenskiy was a White House meeting if he complied, Taylor discovered on 18 July that military aid to Ukraine had been suspended, but it was initially unclear why. Taylor discovered that the hold had been ordered by the White House acting chief of staff, Mick Mulvaney, on the instructions of the president who, Taylor was told, “doesn’t want to provide any assistance at all”.

Taylor said that Sondland had told him the conditions had been laid down to him by Trump personally. Taylor also learned from a senior White House official about a 7 September phone conversation between Trump and Sondland in which the president insisted there was no quid pro quo for Ukraine, but in the same call Trump “did insist that President Zelenskiy go to a microphone and say he’s opening investigations of Biden, and the 2016 election interference”.

Sondland later told Zelenskiy that until he did that, “we will be at a stalemate”.

“I understood ‘stalemate’ to mean that Ukraine would not receive the much-needed military assistance,” Taylor said.

Sondland explained to Taylor “that President Trump is a businessman”.

“When a businessman is about to sign a check to someone who owes him something, he said, the businessman asks that person to pay up before signing the check,” Taylor said.

Taylor also noted that US officials who would normally listen in to phone calls with foreign leaders, were excluded from a call between US diplomats and Zelenskiy in late June, while Sondland ensured that the conversation would not be monitored or transcribed.

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Democratic representative Stephen Lynch, a senior member of the House oversight and reform committee, was quoted in Politico as saying the testimony was a “sea change” that “could accelerate” the impeachment inquiry.

New Jersey representative Tom Malinowski, a Democrat, said it was “the most thorough accounting we’ve had of the timeline”.

Taylor had flown to Washington in response to a congressional subpoena and in defiance of an administration gag order.

The Politico website quoted a source in the private briefing as saying Taylor’s opening statement was 15 pages long and prompted “a lot of sighs and gasps”.

Taylor was brought out of retirement in June after the former ambassador to Kyiv, Marie Yovanovitch, was abruptly removed from her post.

In diplomatic texts released by Congress, it is Taylor who most clearly expresses the quid pro quo being presented to the Ukrainian government by Trump’s emissaries, and his deep concern about it.

“I think it’s crazy to withhold security assistance for help with a political campaign,” Taylor wrote on 9 September to Sondland.

Trump suspended $400m in military aid to Ukraine a few days before a 25 July phone call to Zelenskiy. In the released version of the call, when the Ukrainian president raised the matter of US military support Trump responded: “I would like you to do us a favour though... …”

Trump then asked for the investigations.

Before taking up his post, Taylor requested a meeting with secretary of state Mike Pompeo to seek reassurance about administration support for Ukraine.