A retired US army officer who was a pivotal witness in the first impeachment case against Donald Trump has sued the oldest son of the former president and other Trump allies, accusing them of “intimidation and harassment”.

Alexander Vindman accuses them of participating in an “intentional, concerted campaign of unlawful intimidation and harassment” over his decision to give evidence.

He gave evidence in 2019 impeachment proceedings about a phone call in which Mr Trump pressed his Ukraine counterpart to investigate Democratic rival Joe Biden and his son Hunter.

Mr Trump was impeached by the House of Representatives but acquitted by the Senate in February 2020.

Lt Col Vindman’s lawsuit was filed on Wednesday in federal court in Washington.

It names as defendants Donald Trump Jr; Rudy Giuliani, a long-time Trump adviser and lawyer; and former White House communications officials Dan Scavino and Julia Hahn.

It alleges that after Mr Vindman was summoned by House legislators to give evidence, the defendants and others co-ordinated and advanced “false narratives” about him, including that he was a spy for Ukraine, leaked classified information about him and falsely accused him of lying under oath.

“The actions taken by defendants against Lt Col Vindman sent a message to other potential witnesses as well: co-operate at your own peril,” the suit says, adding that “the message reverberates to this day” as witnesses in the congressional investigation into the January 6 riot at the US Capitol defy subpoenas at the Republican former president’s direction.

Lt Col Vindman announced in July 2020 that he was retiring from the army after more than 21 years and raised similar allegations of harassment, bullying and intimidation as in Wednesday’s lawsuit.

He has since released a book about his experiences, titled Here, Right Matters: An American Story.