One lane is blocked and there's slow traffic due to an accident on the M4 eastbound between J15 A346 Marlborough Road / A419 Marlborough Road (Swindon East) and Membury Services.
Lane three (of three) is blocked.
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The Post Office said it was 'business as usual' but unions said Saturday's industrial action was getting solid support.
Workers and managers are to stage another strike on Saturday amid a long-running dispute over jobs, pensions and closures.
Cloudy in the West with a few showers. Sunny in the East but cold.