The United States ambassador to the European Union is expected to testify before congress that Donald Trump instructed him to help his personal lawyer, Rudy Giuliani, with his plans to pressure Ukraine into investigating Democratic rival Joe Biden.

Gordon Sondland has emerged as a key figure in the Trump administration's pressure campaign to convince Ukrainian president Volodymyr Zelensky to open up an investigation into Mr Biden and his son Hunter.

According to his prepared remarks, Mr Sondland decided to listen to Mr Trump’s direction to work with Mr Giuliani in order to further the ultimate goal of strengthening US-Ukraine relations.

Ordinarily, he will say, he would not have chosen to include a private citizen in matters of US foreign policy.

“Please know that I would not have recommended that Mr Giuliani or any private citizen be involved in these foreign policy matters,” Mr Sondland wrote in the statement.

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“However, given the president’s explicit direction, as well as the importance we attached to arranging a White House meeting between presidents Trump and Zelensky, we agreed to do as president Trump directed,” the statement continued.

Mr Sondland is among several people named in the whistleblower complaint that alleges Mr Trump presented a quid pro quo to Mr Zelensky during a July phone call.