The victim, in her 40s, was hit by a heavy goods vehicle near the Lewknor turn-off this morning (Sunday, January 21).
The crash happened at 4.05am near to junction 6 of the southbound carriageway of the motorway.
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Police described it as “an extremely tragic incident” and have appealed for witnesses.
No arrests have been made.
Investigating officer Sergeant James Mathews of the roads policing unit said: “This is an extremely tragic incident where a woman who had got out of her car, having stopped on the hard shoulder, was involved in a collision with a heavy goods vehicle on the southbound carriageway.
“My thoughts and the thoughts of all of us at Thames Valley Police are with the family of the woman who has died.
“They are being supported by specially trained family liaison officers.
“I am appealing to anybody who was driving on that stretch of the M40 this morning and who witnessed this incident, to please get in touch with Thames Valley Police.
“I would also urge any drivers to check dash-cam and contact us if it has captured the collision.
“You can contact us by making a report online or by calling 101, quoting the reference number 43240031296.”
The M40 southbound was closed for several hours after the crash but has now reopened.