There are six miles of traffic on the A12, which is closed southbound near Chelmsford, Essex, due to the incident. It is believed a lorry has caught fire with some witnesses reporting an "explosion" afterwards.

       Emergency services are at the scene but the exact nature of the incident is yet to be confirmed.

       But Essex Live reports congestion is heavy on both carriageways, with northbound traffic slowing to look at the accident.

       Southbound motorists are being advised to use local roads through Chelmford itself and the town centre is busy as a result.

       Essex Travel News tweeted: "A12 Southbound - blocked due to accident between J19 (Boreham Interchange) and J17 A130 (Howe Green).

       "Congestion to J20A (Hatfield Peverel South), as well as delays Northbound due to onlookers."

       Long delays remain.

       Firefighters were called to a vehicle fire on the A12 at Boreham on the southbound carriageway at 1:37pm.

       A spokesman said: "On arrival, crews reported that a mobile crane, approximately 35 tonnes, had an engine fire and firefighters worked to extinguish the fire by 2:23pm.

       "Crews currently remain at the scene to make the area safe and clean spilled hydraulic fluid following the fire.”