A fire involving a lorry carrying hay bales has caused the closure of the M4 in both directions between junction 16 and 17.
The closure is expected to remain in place for a considerable time and there are long delays.
Swindon Police said: "We are supporting colleagues from the fire service at the scene.
"Please seek an alternative route."
The fire service has advised the incident is a major hay fire.
National Highways South-West tweeted: "Load keeps reigniting so unsafe to run traffic past scene until under control.
"We don't have a clear timescale to do this.
"If you're in the closure thanks for your patience. We will get you our in due course."
UPDATE: Trapped vehicles will be released in one lane in about an hour, according to National Highways South-West.
The M4 between junctions 16 and 17 will remain closed for a considerable time.
They tweeted: "We appreciate it is frustrating having. towait but we all want to ensure the fire is fully under control before letting the public drive past."
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