Harry Dunn's parents have dropped their legal claim against Northamptonshire Police after concluding the force are "absolved of any blame" following their son's death.
The 19-year-old's family are set for a two-day judicial review hearing at the High Court in November where they allege the Foreign Secretary "obstructed justice" by allowing suspect Anne Sacoolas to leave the UK.
Northamptonshire Police were brought into the legal claim in January as Mr Dunn's mother Charlotte Charles and father Tim Dunn sought to understand how Sacoolas was able to claim diplomatic immunity following the fatal crash in August last year.
Pictures of the week: July 19 - 25
Pictures of the week: July 19 - 25
Mothers are seen chatting on a playground on a sunny day as England works its way to limit the spread of Coronavirus, Slough, England July 23, 2020. England managed to lower daily Covid-19 cases from several thousand to hundreds thanks to the lockdown it introduced. From tomorrow Brits will be obligated to wear face protective masks in shops. (Photo by Dominika Zarzycka/NurPhoto via Getty Images)
A Primark security guard poses for a photo wearing a face mask outside Primark in York city centre as government guidance has made face coverings mandatory in shops and supermarkets in England to prevent the spread of the ongoing coronavirus disease
WOKINGHAM, ENGLAND - JULY 23: Georgia Hall of England plays her tee shot on the 14th hole during The Rose Ladies Series at Bearwood Lakes on July 23, 2020 in Wokingham, England. (Photo by David Cannon/Getty Images)
A woman in a protective face mask is seen leaving a shop as England works its way to limit the spread of Coronavirus, Slough, England July 23, 2020. England managed to lower daily Covid-19 cases from several thousand to hundreds thanks to the lockdown it introduced. From tomorrow Brits will be obligated to wear face protective masks in shops. (Photo by Dominika Zarzycka/NurPhoto via Getty Images)
LONDON, UNITED KINGDOM - 2020/07/23: Social Distancing Ambassadors employed by Veolia for the safe reopening of Londons West End to help visitors safely transition out of lockdown, and advise people should they fail to socially distance themselves as the wander around or correctly wear their facemasks in the UK's busiest shopping and entertainment district. (Photo by Dave Rushen/SOPA Images/LightRocket via Getty Images)
Britain's Prime Minister Boris Johnson holds crabs caught on the Carvela with Karl Adamson at Stromness Harbour in Stromness, Orkney, Scotland, during a visit to the Highlands and Northern Isles of Scotland, Thursday July 23, 2020. (Robert Perry/PA via AP)
CORRECTS NAME OF REGIMENT A young visitor imitates the march of a guardsman from the King's Royal Hussars, at Windsor Castle which has reopened to the public after the lifting of further coronavirus lockdown restrictions in England, Thursday July 23, 2020. The COVID-19 restrictions are eased Thursday to allow some tourist attractions to open their gates for public visitors. (Jonathan Brady/PA via AP)
Mayor of London Sadiq Khan, centre, tries out a new Streetspace protected cycle lane in London, during the launch of a new online cycle training scheme, Thursday July 23, 2020. The online cycle training course is launched to encourage London's ‘green recovery’ from coronavirus, offering help for participants to get their bike ready for their first ride and offers tips for avoiding potential hazards. (Stefan Rousseau/PA via AP)
Student driver Roisin Clarke, 17, has a debrief with instructor Kayleigh Nurse (right), after passing her driving test in Yeovil, Somerset. Driving tests, which were suspended across the UK in March, have restarted in England with a huge backlog after hundreds of thousands of tests were delayed or cancelled due to the coronavirus lockdown.
LIVERPOOL, ENGLAND - JULY 22: Jordan Henderson of Liverpool holds the Premier League Trophy aloft as the team celebrate winning the League during the presentation ceremony of the Premier League match between Liverpool FC and Chelsea FC at Anfield on July 22, 2020 in Liverpool, England. Football Stadiums around Europe remain empty due to the Coronavirus Pandemic as Government social distancing laws prohibit fans inside venues resulting in games being played behind closed doors. (Photo by Laurence Griffiths/Getty Images)
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NEWCASTLE UPON TYNE, ENGLAND - JULY 22: Mikko Korhonen of Finland and his caddie Nick Mumford look to play a shot on the 4th hole during Day 1 of the Betfred British Masters at Close House Golf Club on July 22, 2020 in Newcastle upon Tyne, England. (Photo by Andrew Redington/Getty Images)
A photographer lines up his shot at sunrise at St Mary's Lighthouse in Whitley Bay in the early hours of Tuesday morning. Picture date: Tuesday July 21, 2020. Photo credit should read: Owen Humphreys/PA Wire (Photo by Owen Humphreys/PA Images via Getty Images)
LONDON, ENGLAND - JULY 21: Dame Vivienne Westwood is suspended 10 Feet high inside giant bird cage in protest for Julian Assange at Old Bailey on July 21, 2020 in London, England. Dame Vivienne Westwood, re-entering public life for the first time after having been shielding for 16 weeks during the COVID-19 lockdown, suspends in giant birdcage in front of The Old Bailey Criminal Court, in protest about the illegal U.S. extradition of Julian Assange for telling the truth about American war crimes. (Photo by Gareth Cattermole/Getty Images)
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Fashion Designer and activist Dame Vivienne Westwood takes part in a protest which sees her suspended 10ft high inside a giant bird cage outside The Old Bailey (Central Criminal Court of England), in protest against the extradition of Julian Assange. (Photo by Keith Mayhew / SOPA Images/Sipa USA)
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LEEDS, ENGLAND - JULY 19: Leeds United fans celebrate after winning the Sky Bet Championship title at Millennium Square on July 19, 2020 in Leeds, England. (Photo by George Wood/Getty Images)
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The PA news agency understands the force will now only be involved in the claim as an "interested party" on the request of Chief Constable Nick Adderley.
It is also understood the force will not seek costs against the family for its involvement in the proceedings.
The Dunn family has decided to discontinue proceedings against the police after alleging that documents disclosed for court proceedings demonstrated the force was "kept in the dark" by the Foreign Office (FCO).
Recent documents, seen by PA, showed that Northamptonshire Police said it was "not informed" by the FCO of the arrangements that allowed Sacoolas to claim diplomatic immunity.
The force also claims the FCO did not tell it about the risk of Sacoolas being withdrawn from the UK or the existence of a waiver in respect of her husband Jonathan's immunity – preventing officers from interviewing him as a witness.
In December last year, Sacoolas was charged with causing Mr Dunn's death by dangerous driving but an extradition request submitted by the Home Office was rejected by US Secretary of State Mike Pompeo in January.
The US State Department described the decision as "final" but the UK and the US have since agreed to amend the "anomaly" which allowed Sacoolas to claim diplomatic immunity.
Explaining why the parents have dropped the claims against Northamptonshire Police, family spokesman Radd Seiger told PA: "Having carefully reviewed the documents disclosed by the Foreign Secretary, the parents are now satisfied that the police are absolved of any blame and could have done no more to ensure that Mrs Sacoolas was brought to justice.
"Our case remains that the documents clearly show how the police investigation was effectively stopped in its tracks abruptly when the Foreign Office told the police shortly after Harry died that Mrs Sacoolas had diplomatic immunity.
"We say the documents reveal that the Foreign Office kept the police in the dark for 14 days about the uncertainty surrounding Mrs Sacoolas' claim to immunity.
"They failed to tell them that Mrs Sacoolas was planning to leave the country, and then did not tell the police that she had gone until the day after she left.
"In the United Kingdom, it is the police's responsibility to establish who has diplomatic immunity, not the Foreign Office, yet he and his team did just that – the parents say unlawfully.
"In discussions with the police, they have now graciously confirmed that they will remain in the legal case as an interested party."
A Northamptonshire Police spokesman said: "This is an ongoing legal matter and it would therefore be inappropriate to comment further at this time."
The Foreign Office have been approached for comment.