Transcripts released on Saturday in the impeachment inquiry show the US Ambassador to the EU, Gordon Sondland, playing a key role in President Donald Trump’s effort to have the country conduct political investigations as a condition for receiving military aid.

The fresh details come from hundreds of pages of testimony from Tim Morrison, a former prominent official at the National Security Council (NSC).

The testimony contradicts much of the ambassador’s own testimony behind closed doors. Mr Morrison and Mr Sondland are expected to testify publicly before the House next week.

While some, including Mr Trump himself, have begun to question Mr Sondland’s knowledge of events, Mr Morrison told House investigators the ambassador “related to me he was acting — he was discussing these matters with the President”.

Morrison, a longtime Republican defence hawk in Washington, largely confirmed testimony from current and former officials testifying in the impeachment inquiry, which sat for a rare Saturday session.

But his account also provided new insight on what others have called a shadow diplomacy being run by Mr Trump’s personal lawyer, Rudy Giuliani, which is often at odds with US national security interests.

As Mr Sondland, Mr Giuliani and others tried to persuade new Ukraine President Volodymyr Zelenskiy to launch the investigations Mr Trump wanted into his Democratic rivals, Mr Morrison said he “tried to stay away”.

Mr Morrison called this the Burisma “bucket” — investigations into the family of Joe Biden and the role of Democrats in the 2016 election. It is a reference to Burisma, the gas company in Ukraine where Mr Biden’s son Hunter served on the board.

In particular, Mr Morrison described a September 1 meeting Mr Sondland held with a top Zelenskiy aide, Andriy Yermak, on the sidelines of a summit in Warsaw.

Mr Morrison said he witnessed the exchange and that afterwards Mr Sondland bounded across the room to tell him what was said.

Mr Sondland told him that “what could help them move the aid was if the prosecutor general would go to the mike and announce that he was opening the Burisma investigation,” Mr Morrison testified. The prosecutor general is Ukraine’s top legal official.

“My concern was what Gordon was proposing about getting the Ukrainians pulled into our politics,” Mr Morrison said. “It was the first time something like this had been injected as a condition on the release of the assistance.”

Mr Morrison, who announced on October 30 he would be stepping down from the NSC, was brought to the White House by then-national security adviser John Bolton.

Within hours of the conversation in Warsaw, Mr Morrison called Bolton and the top US official in Ukraine, William Taylor. He told them both about the conversation and his concerns about it.

Mr Bolton had told him: “Stay out of it, brief the lawyers.”

For weeks, top administration aides had been struggling to understand why the $391 million (£303 million) in security aid for Ukraine was being delayed. There had been long-standing bipartisan support for backing up the young democracy bordering an aggressive Russia.

Others have testified they were being told by officials at the Office of Management and Budget it was being stalled at the direction of the president’s acting chief of staff, Mick Mulvaney.

A few days later, on September 7, Mr Sondland rang Mr Morrison to tell him he had just finished a call with the president.

“I remember this because he actually made the comment that it was easier for him to get a hold of the President than to get a hold of me,” Mr Morrison said.

Mr Morrison said Mr Sondland related that Mr Trump had assured him there were no strings being attached to the military aid for Ukraine.

“The president told him there was no quid pro quo, but President Zelenskiy must announce the opening of the investigations and he should want to do it,” Mr Morrison recalled Mr Sondland saying.

Only days later, after three congressional committees said they were launching inquiries into efforts by Mr Trump and Mr Giuliani to investigate the Bidens, was the money released.

Mr Morrison said that at a September 11 meeting at the White House, Mr Trump was persuaded to release the money.

Vice President Mike Pence and Senator Rob Portman of Ohio “convinced the president that the aid should be disbursed immediately,” said Mr Morrison, who was briefed about the meeting but did not attend.

“The case was made to the president that it was the appropriate and prudent thing to do.”

Mr Trump, who says he only wanted to root out corruption in Ukraine, says he did nothing wrong.