Traffic chiefs said at noon today the smash happened northbound between junction 31 for Preston, Blackburn, Clitheroe and the A59, and junction 31A for Preston East and Longridge.
Motorway cameras showed queuing traffic, while traffic maps reported delays of around 20 minutes, with vehicles travelling at an average speed of just 10mph at around 12.25pm.
While the cause of the smash was unknown, it happened during heavy rain.
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