The Independent Police Complaints Commission (IPCC) has been called in after a collision on the M4 at Newport in which five people died.
Five people have died after a car which had been pursued by police crashed head on with another vehicle on the M4.
A sixth person suffered multiple injuries in the crash at 0320 BST which has shut the motorway both directions from junction 24 to 28.
The Independent Police Complaints Commission (IPCC) has been called to investigate the crash which happened at 0320 BST at Newport.
Three young men in one car died at the scene and a fourth is in hospital, the Welsh Ambulance Service confirmed.
Police said officers stopped following the car when it turned onto the motorway in the wrong direction.
A couple - a man and a woman in the second car - also died at the scene.
Three men in one car died at the scene and a fourth is critical. A couple in the second car also died.
The collision between a Red Ford Mondeo and a Red Volvo 940 happened happened on the eastbound carriageway between junctions 25 and 24 in Newport.
The collision between a red Ford Mondeo and a red Volvo 940 happened on the eastbound carriageway between junctions 25 and 24.
The IPCC has said it has been asked to carry out an investigation.
Gwent Police have said the M4 will be closed for some time
Gwent Police have said the M4 will be closed for some time
A spokesman said: "I can confirm that Gwent Police have referred the incident to us.
The westbound carriage of the motorway was reopened just after 1200 BST but the eastbound carriageway is still closed from junction 24 to 28.
"I haven't got any further details but I know it is on the grounds that, prior to the incident, there was some form of police involvement."
Earlier on Monday, the IPCC confirmed that it had been asked to carry out an independent investigation.
A spokesman for Gwent Police said the identification of those killed is "currently being ascertained and theirfamilies informed".
IPCC Commissioner Tom Davies said: "It has emerged that at around 0300 this morning a marked police vehicle had reason to try and stop a car containing four male occupants.
It is thought the M4 will remain closed for some time between J28 (Coldra) and J24 (Tredegar Park) eastbound and J24 and J26 (Malpas Road) westbound.
"Early indications are that the vehicle being pursued took a turn onto the motorway in the wrong direction.
There are currently tailbacks of up to five miles.
"The police vehicle did not follow it. Some one-and-a-half miles down the motorway, the fatal collision occurred."
A police spokesman said: "Every effort is being made to open up the carriageways of the M4 as soon as possible."
A spokesman for Gwent Police said the identification of those killed is "currently being ascertained and their families informed".
Westbound traffic is being diverted off at Junction 24. Eastbound traffic is being taken off onto the distributor road at J28. Seven mile tailbacks are reported back to J30 (Cardiff Gate).
Monday morning rush house traffic was tailing back for up to seven miles at one point with all main roads across south east Wales heavily congested.
There is also heavy traffic on all main roads around the motorway.
Eastbound traffic is being taken off onto the distributor road at J28.
Last month the M4 was closed around Newport for many hours after a lorry driver died in an accident at Tredegar Park.
Last month the M4 was closed around Newport for many hours after a lorry driver died in an accident at Tredegar Park.
At their worst, tailbacks for drivers after that crash reached 10-miles.Are you caught up in the accident? Please send us your comments using the form below.
At their worst, tailbacks for drivers after that crash reached 10-miles.Are you caught up in the accident? Please send us your comments using the form below.
