Thames Valley Police has been unable to publicly confirm many details at this stage, but passersby say they've been left 'totally shaken' at the scene. One eyewitness described the incident to the Swindon Advertiser as a "truly horrific scene", adding: "We drove past this before anybody was on the scene. The M4 eastbound has been closed past Swindon (Image: National Highways)A spokesperson for Thames Valley Police said: "We are currently dealing with a road traffic collision on the M4. The M4 eastbound has been closed past Swindon (Image: National Highways)Eastbound traffic is being diverted off the motorway north onto the A419 and south towards Marlborough. Wiltshire Police has stated that Thames Valley Police is leading the response, and emergency services remain at the scene.