A landslip onto the M4 caused by heavy rain has led to traffic delays and flooding on the eastbound carriageway.
Deep water next to the collapsed bank reduced traffic flow to one lane between junctions 12 and 13.
The Highways Agency is advising motorists to avoid the area after severe weather caused floods and road problems across Berkshire.
The A34 at Chieveley has been closed because of flooding at the point where it passes under the M4 at junction 13.
Diversions are in place and traffic is moving slowly.
A Highways Agency spokesperson said: "At this stage the M4 is not closed but there may be some obstructions where debris has come out of the drains.
"There are quite a lot of incidents on the road and that is causing congestion."
She added: "Our advice is to avoid the area if possible."
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