An articulated lorry carrying bales of hay caught fire on the London-bound motorway between Faversham and Sittingbourne, in Kent, at 10:15 BST.
It is currently closed to traffic with reports of seven-mile tailbacks. There are diversions via the A20 and M20.
Highways England said the motorway was expected to reopen at about 23:00 BST.
No-one is believed to have been injured in the incident.
Kent Police tweeted that six motorists driving on the hard shoulder would be prosecuted after the fire engine "couldn't get through".
It advised that if stuck on a motorway, drivers should wait and "we will get you out if it is safe".
Six drivers are to be prosecuted for driving on the hard shoulder of the M2 after a fire engine trying to reach a lorry in flames was held up.