Emergency services were alerted when a silver Ford Fiesta Zetec crashed into a tree on the road near Bullocks Sawmill, north of Hexham.
A Northumbria Police spokesperson said: “It was reported that the silver Ford Fiesta Zetec was travelling northbound on the A68 when – for reasons to be established – it has crossed onto the southbound carriageway and collided with a tree.
“Emergency services attended and the female passenger of the vehicle – aged 87 – had suffered life-threatening injuries and sadly passed away.
“The 88-year-old male driver of the vehicle was taken to hospital with serious however non-life-threatening injuries.
“The woman’s next of kin has been informed and they are currently being supported by officers at this difficult time.
The spokesperson added: “An investigation was launched by the Force’s Motor Patrols team to establish the full circumstances surrounding the collision – and officers are appealing to the public for information.
“A number of witnesses have already been spoken to, but officers are now asking anyone else who is yet to come forward to get in touch with police.
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“Our thoughts go out to her family and loved ones at this devastating time, and we will continue to support them in every way we can.
“We are especially keen to hear from any passing motorists or witnesses who might have saw what happened, or have any CCTV or dashcam footage.”
Anyone with information is asked to contact Northumbria Police using the ‘Tell Us Something’ page of the website, or call 101, quoting log number NP-20230725-0885.