A serious crash on the M4 eastbound between junctions 5 and 4B is causing severe delays.
The incident has forced the Highways Agency to close all four lanes, with the road not expected to reopen again until 6pm.
The collision is also causing delays on the westbound carriageway, with the Highways Agency saying there will be moderate delays until at least 5pm.
Four vehicles, including a lorry, have been involved in a serious road traffic accident on the M4 between junctions 5 and 4B.
Police arrived on the scene just before 4pm this afternoon and the carriageway has been closed in both directions to allow access for the emergency services.
Motorists are being advised to avoid the area as it is likely to be shut for the rest of the day.
Twitter user Matt Freeth has posted this picture of the air ambulance arriving at the scene of the accident:
Thames Valley Police says the motorway will remain closed eastbound between junctions 5 and 4b for at least the next two hours.
Westbound has been temporarily opened again.
The Highways Agency has confirmed that the emergency services are on the scene and a police investigation is underway, with the closure of the eastbound carriageway expected to last for several hours.
Currently there are delays of more than 30 minutes westbound.
Drivers are advised to avoid the area if possible, if not then to follow the diversion marked by the hollow diamond symbol- leaving the M4 at junction 6 and travelling via the A4, A3044 and A3113 to the M25 clockwise at junction 15.
Two lanes have now been reopened on the eastbound carriageway, but the Highways Agency is still warning of long delays.
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