The M5 Southbound between junctions 25 and 26 (Taunton) is expected to stay closed until at least 11am.
Motorists are being advised to take alternative routes.
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Parents have said they are pulling their children out of school for the day in strike action against the government.
A phone line mix-up has caused chaos in a village - after residents were left with their neighbours's numbers.
Among the cyclists is Andrew Venn whose son Joe has a congenital heart condition and will need hospital treatment all his life.