Three people have been injured, one seriously, after a "serious multi-vehicle accident" on the M40 near High Wycombe.
The motorway was initially closed in both directions between Junction 4 and Junction 5 for a "police-led incident" that had "all emergency services on scene".
By 4.30pm, Thames Valley Police said the southbound carriageway had partly re-opened but all northbound lanes remained closed.
At around 2.10pm officers were called to reports of a two car collision on M40 northbound between junction 4 and 5, the force said in a statement.
All those injured have been taken to hospital.
Investigating officer, PC Chris Fallon of the Joint Operations Roads Policing Unit based at Taplow, said police were aware "a number of vehicles may have been travelling at speed" as he called for witnesses
“We are appealing for any witnesses to this collision to please come forward," he said.
“Additionally we are asking anyone who has a dash-cam and was travelling on the M40 at around 2.10pm between junctions 4 and 5 in either direction to check their footage and please get in touch if you think it will aid our investigation.
“We are aware that a number of vehicles may have been travelling at speed on this stretch of the M40 prior to this incident and we are asking anyone who may have seen this to please also get in touch.”
At the time of the accident, a motorist stuck in traffic said there were four to five ambulances, a helicopter and police cars on the scene.
Highways England confrimed that an air ambulance helicopter had landed on the northbound carriageway.
Oxford Tube tweeted to say the motorway was closed after a "serious multi-vehicle accident".
Liz Saul tweeted: "At a standstill for more than 30 min on #M40 southbound between 5 & 4 due to an rtc. Seen emergency services & @HighwaysEngland officers go by. Air ambo possibly been hovering too."
Anyone with information should call 101 quoting reference 710 23/02/20.
