The westbound exit ramp at Junction 15 was closed for emergency repairs after the road surface was badly damaged by the coach fire.
Dramatic footage shows firefighters tackling the blaze as smoke billows around the charred vehicle.
It is understood that everyone on the coach left the vehicle safely.
National Highways said: “The #M4 J15 westbound off-ramp near #Swindon will remain closed overnight for emergency road surface repairs following extensive damage from a coach fire .”
The ramp has been closed for about 3 p.m.
All four lanes of the freeway were originally closed but have now reopened.
In an update, the INRIX Traffic Department said: “The M4 Westbound exit slip road has been closed due to a coach fire at J15 A346 Marlborough Road (Swindon East).
“The exit slip road has remained closed since 2.55pm, The M4 had been held briefly to allow police to clear traffic stuck on the exit slip road, all lanes have reopened on the main road.”
Motorists caught in traffic took to Twitter to post about the incident.
Twitter user Terry King shared a photo showing the coach on fire and warned of traffic jams.
He said: ‘The coach fire at Junction 15 appears to be out and construction of safe traffic from all angles is to be avoided.
Dorset and Wiltshire Fire Services urged drivers to use alternative routes in a tweet at 3.30pm.
He said: “We are currently dealing with a vehicle fire on the westbound M4 near J15.
“It’s impacting local traffic, so take an alternate route if you can.”
Express.co.uk has contacted Dorset and Wiltshire Fire Services and Wiltshire Police for comment.
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This notice was published: 2022-05-05 18:35:00