Investigators should take Michael Flynn's reported offer of testimony in exchange for immunity from prosecution "if he tells all of the truth about Russia/Trump", a former White House ethics chief has said.

Mr Flynn, Donald Trump's former national security adviser who resigned after less than a month over his undisclosed contact with a Russian diplomat, reportedly offered on Thursday to cooperate with the wide-ranging investigation of alleged links between the Kremlin and Mr Trump's Presidential campaign if he is given immunity from prosecution.

Ethics expert Richard Painter said on Twitter: "Flynn offers testimony for immunity. FBI/DOJ should take the deal if he tells all of the truth about Russia/Trump. [It] should be negotiated by a professional INDEPENDENT prosecutor not Sessions DOJ."

Mr Painter, who served as George W Bush's chief ethics lawyer from 2005 to 2007, later added: "Renaming my kids' pet canary bird General Flynn."

Norm Eisen, who performed the same role for Barack Obama between 2011 and 2014, said: "Have been predicting 4days that Flynn will roll over on others. Who/what will he trade for immunity? Trump hisself?"

Footage of Mr Flynn saying that striking an immunity deal means "you've probably committed a crime" resurfaced on Friday morning.

Speaking to NBC's Chuck Todd about Hillary Clinton's email controversy, he said: "The very last thing that [Democratic campaign manager] John Podesta just said is, 'No individual too big to jail'. That should include people like Hillary Clinton.

"I mean, five people around her have been given immunity, to include her former chief of staff. When you are given immunity that means that you've probably committed a crime."

On Thursday night Mr Flynn's lawyer, Robert Kelner, released a statement saying his client "certainly has a story to tell, and he very much wants to tell it, should the circumstances permit".

Mr Kelner defended Mr Flynn as a "highly decorated" veteran who had "devoted most of his life to serving his country".

"Notwithstanding his life of national service, the media are awash with unfounded allegations, outrageous claims of treason, and vicious innuendo directed against him. He is now the target of unsubstantiated public demands by members of Congress and other political critics that he be criminally investigated," he added.

"No reasonable person, who has the benefit of advice from counsel, would submit to questioning in such a highly politicised, witch-hunt environment without assurances against unfair prosecution."

The 58-year-old retired US Army lieutenant general found himself at the centre of a gathering storm in February after it emerged he had spoken with a Russian diplomat about the issue of US sanctions before Mr Trump took office, and indicated the relationship between the two countries would improve under a new administration.

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Sanctions had been imposed by Barack Obama in response to Russia’s alleged cyber-interference in the presidential election.