Traffic is building up following an accident just outside of Maidenhead.
The M4 is reportedly closed westbound from the town following an earlier accident.
AA Traffic News is reporting delays of up to 65 minutes with an average speed of 6mph.
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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.