An on-duty police officer and a 91-year-old woman have died following a crash on the A4 in Berkshire.
PC James Dixon died after the motorcycle he was riding was in collision with a car on Bath Road near Hare Hatch at 13:50 GMT.
The pensioner, who was a passenger in the car, was killed while the driver, also a woman, was taken to hospital.
Officers said the road would remain closed for the remainder of Tuesday.
Thames Valley Police said the next of kin of both PC Dixon and the deceased woman have been informed.
A force spokesman said officers remained at the scene of the collision and had advised motorists to avoid the area.
The incident has been referred to the Independent Police Complaints Commission (IPCC).
IPCC Associate Commissioner Guido Liguori said: "My thoughts and sympathies are with their families and friends and the colleagues of the officer at this very difficult time.
"IPCC investigators are attending the scene as part of an independent investigation to determine the circumstances which lead to the collision."
PC Dixon was based at Loddon Valley police station, near Reading.
Police said the injuries of the driver involved are "not thought to be life threatening".
Tributes to PC Dixon have been posted in comments on Thames Valley Police's Facebook page.
Daniel Ruffle said: "Ride the sky and the clouds big man, it was a pleasure knowing you and working with you."
Bernadette Ellison said: "God bless you Dixie, you made me laugh with your wicked sense of humour."
An on-duty police officer who died along with a 91-year-old woman in a crash on the A4 in Berkshire was due to become a father.
PC James Dixon, whose wife Samantha is heavily pregnant, died when his motorcycle was in collision with a car near Hare Hatch on Tuesday.
The 39-year-old had appeared in the Sky TV programme Road Wars.
The pensioner, who was a passenger in the car, was also killed while the female driver was taken to hospital.
The incident, which happened on Bath Road, has been referred to the Independent Police Complaints Commission (IPCC).
PC James Dixon died after the motorcycle he was riding was in collision with a car on Bath Road near Hare Hatch
The IPCC said PC Dixon was on a training exercise and was not responding to an emergency or pursuing the vehicle he collided with.
He was also not involved in any other pursuit and his unmarked police bike did not have flashing lights or sirens on.
PC Dixon, known as "Dixie" to friends, was based at Loddon Valley police station, near Reading.
Prime Minister Theresa May paid tribute to PC Dixon and the elderly woman during Prime Minister's Questions on Wednesday.
She said: "I'm sure the whole house will wish to join me in offering condolences to the family, friends and colleagues of police constable James Dixon from Thames Valley Police who was killed while on motorcycle duty yesterday..."
Tributes to PC Dixon have been posted in comments on Thames Valley Police's Facebook page, with many referring to watching him on the Sky 1 TV series Road Wars.
PC James Dixon was a "hugely respected" officer and had served 18 years "in a variety of roles" in the force
Thames Valley Chief Constable Francis Habgood said PC Dixon was a "hugely respected" officer and had served 18 years "in a variety of roles" in the force.
All the force's flags will fly at half-mast for the next week as a mark of respect to PC Dixon.
Jo Gill said: "Goodnight Dixie, you really were one of the funniest and best blokes I worked with. You are un-replaceable and my heart goes out to Your family both blood and blue."
Dave Bulger added: "Stand down Dixie, I can't believe you're gone mate. It was a real pleasure to know you and my heart goes out to your family and loved ones."
PC James DixonImage copyrightSKY TV
PC James Dixon appeared in the Sky TV programme Road Wars
On Twitter, Ryan Sheehan said: "Growing up I was a big fan of PC James 'Dixie' Dixon and his @tvprp colleagues as they featured in #RoadWars.
"It's well respected officers like PC Dixon that inspire me to be a police officer..."
Road Wars followed 14 members of Thames Valley Police's roads policing officers from 2003 to 2010.
PC Dixon, who grew up in Winchester, is believed to have taken part in several series of the programme and was filmed with his partner "Yorkie".
Do the police use the pre-crash speed, revs, gear etc. information from the black box or event data recorder? I suspect it might mysteriously be unusable in this case.
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