In comments that appeared to debunk one of the conspiracy theories Republicans have often cited to try and undermine the former vice president, Kurt Volker told member of Congress he did not believe it.
“As I said, I don’t find it plausible or credible that vice president Biden would have been influenced in his duties,” said Mr Volker told the House Intelligence Committee.
“But Ukrainians in the society we know…Ukraine has been for decades…trying to act in a corrupt way or buy influence, that is plausible.”
The comments from Mr Volker, a former US special envoy to Ukraine and a one-time ambassador to Nato, were all the more striking because he was among those witnesses called by Republican members of the committee.
Mr Volker also highlighted the role Donald Trump’s personal lawyer, Rudy Giuliani, played in pushing the administration’s policy towards Ukraine.
He said Mr Giuliani mentioned allegations about Mr Biden and his son, Hunter, during a meeting, and that the former New York mayor had “stressed that all he wanted to see was for Ukraine to investigate what happened in the past and apply its own laws”.
Mr Volker went on to say he told Mr Giuliani he did not think Mr Biden would have been influenced in any way by financial or personal motives.
Republicans have long alleged Mr Biden used his position as vice president to oust a Ukraine prosecutor who was tasked with investigating corruption. They claim Mr Biden did so as part of an effort to protect his son, who had a position on the board of Burisma, a Ukraine-based energy company.
Mr Biden has always denied the claims and there has been no evidence produced to support them.
“At the one in-person meeting I had with mayor Giuliani on July 19, mayor Giuliani raised, and I rejected, the conspiracy theory that vice president Biden would have been influenced in his duties as vice president by money paid to his son,” Mr Volker said. “As I testified previously, I have known vice president Biden for 24 years. He is an honourable man and I hold him in the highest regard.”
Mr Volker claimed he had not initially not realised Mr Trump was holding up military aid to Ukraine to pressure the country to investigate Mr Biden’s son, and why he had been appointed to the board of Burisma.
“In retrospect I should have seen that connection differently, and had I done so, I would have raised my own objections,” Mr Volker said.
“In hindsight, I now understand that others saw the idea of investigating possible corruption involving the Ukrainian company, Burisma, as equivalent to investigating former vice president Biden. I saw them as very different. The former being appropriate and unremarkable, the latter being unacceptable”
He added: “In retrospect, I should have seen that connection differently, and had I done so, I would have raised my own objections.”
Democrats launched an impeachment investigation after a whistleblower, believed to be a member from the US intelligence community, alleged that in his call to Ukraine leader Volodymyr Zelensky, Mr Trump had requested he launch an investigation into Joe Biden and his son, in exchange for the release of military aid and a state visit to the US.
Mr Trump, who has denied any wrong doing, has described the investigation as a “witch hunt”.