The shadow foreign secretary has come to the support of Harry Dunn's family by raising further questions about her counterpart Dominic Raab.
Senior Labour Party MP Emily Thornberry branded the foreign secretary 'either a liar or incompetent' over his handling of the Anne Saccolas scandal in The Mail on Sunday.
Labour supports the Northamptonshire family's calls for a public inquiry into the diplomatic aftermath of the fatal crash near Croughton last year after it was revealed the American driver used to work for the CIA.
Ms Thornberry told the national paper: "While you're not allowed to call a fellow MP a liar in the House of Commons, all I'll say is that Dominic Raab either hadn't bothered to find out the truth about Anne Sacoolas before his October statement to Parliament or knew the truth but chose not to share it.
"That is exactly why I agree with the Dunns in calling for an urgent public or parliamentary inquiry into the handling of Harry's case."
Harry, 19, died in hospital after being involved in a crash with Sacoolas' Volvo while riding his motorcycle near RAF Croughton in August 2019.
Sacoolas refuses to return to the UK to face despite being charged with causing death by dangerous driving and admitting to driving on the wrong side of the road.
Ms Thornberry said the Foreign and Commonwealth Office (FCO) would be guilty of gross incompetence if they did not know Sacoolas had a history with the CIA until after she left the country.
But if the government knew beforehand, then they owe the family an apology and explanation, the Labour MP said.
In response, Dunn family spokesman Radd Seiger tweeted: "The innocent and bereft family of Harry Dunn were lied to, misled, and had vital information withheld from them by UK authorities in the weeks and months after the crash which continues to this day.
"We will expose misconduct and the subsequent scandalous attempt at a cover-up."
The FCO insists Sacoolas was notified to the government as a spouse with no official role and there are no plans to hold an inquiry into the case.