The motorway has shut between junction 15 and 16 near the M40 since 7.30am today (August 16).
National Highways (@HighwaysEAST) is urging motorists on X (formally known as Twitter) to consider alternative routes where possible.
There are currently five miles of congestion on approach which is causing 60-minute delays.
All lanes were closed as the incident was dealt with however two of the three lanes have now reopened.
One lane remains closed for recovery work.
We have contacted Thames Valley Police and South Coast Ambulance Service for further information.
