Delays are affecting traffic travelling westbound on the M4 between junctions 7 and 8/9 near Maidenhead.
A police spokeswoman said there was a collision between vehicles, but no one was hurt.
The damaged vehicles have been moved to the hard shoulder but traffic is still moving slowly.
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The Met Office has issued a yellow alert for Windsor and Maidenhead, Slough and Wokingham, warning that rain moving eastwards across the south of the country may turn to snow in some places.