A man was arrested on suspicion of causing a serious injury by dangerous driving. He remains in custody at Gablecross Police Station.
One lane on the eastbound carriageway is now re-opened and, it is hoped, that all lanes on both carriageways will be fully re-opened within the hour.
As a result of these collisions, long tailbacks have been caused on the M4 and the A419 at the Commonhead roundabout, Swindon.
Motorists are advised to avoid the area if possible and, advice for any motorists in the area is to remain calm and inside your vehicle.
Anyone with information on this incident is asked to contact Wiltshire Police on 101 or Crimestoppers on 0800 555 111 where information can be left anonymously.