Two men were engulfed in a fireball and badly injured when a van carrying petrol-filled industrial machinery exploded on a busy bypass.

Police suspect the fire, which caused chaos for hundreds of motorists on the A23, was sparked when one of the men lit a cigarette, igniting petrol vapours inside the van.

Both men suffered severe burns and were last night being cared for at a specialist burns unit at the Royal Victoria Hospital in East Grinstead. Their injuries were serious but not thought to be life-threatening.

The Mercedes Sprinter van, belonging to Beresfords Flooring, based in Ashbourne, Derbyshire, was engulfed in flames as it travelled northbound along the A23 at Albourne at 3.55pm yesterday.

The men, whose clothes and bodies caught fire, managed to drive the van off the road and in to a ditch before escaping and rolling around in the grass to put out the flames.

No other vehicles were involved in the fire but motorists were caught in long tailbacks in both directions of the A23.

Safety airbags used in construction work and four leaf-blowers which were filled with petrol were in the back of the van.

Police and firefighters suspected the petrol ignited when a cigarette was lit.

It was not known how much petrol was in the machinery but vehicles may only legally carry ten litres of petrol in two containers.

Sergeant Lawrence Culver, based at Haywards Heath's road policing unit, said the men had told officers there was a strong smell of petrol in the van before the blaze.

He said: "We believe one man is suffering from ten per cent burns and the other man from 15 per cent burns but both are very lucky to be alive."

Firefighters from East and West Sussex Fire and Rescue Services arrived at the scene and closed the road for several hours.

A spokesman for the fire service said: "There was a lot of petrol in the van. The vehicle was doing 70mph, Both men managed to get out but were on fire.

"Who in their right mind if they can smell petrol in a vehicle would light up a cigarette?

Scores of motorists were diverted along the A281 near Henfield as rush hour began.

Many were caught for hours in long tailbacks as the emergency crews tended the victims and investigated the incident.

The northbound carriageway of the A23 remained closed for hours but queues also built up on the southbound carriageway.

Southbound tailbacks stretched more than 15 miles, as far back as East Grinstead, as drivers slowed down to rubberneck at the scene.

By 6.45pm the road was reopened and the remains of the burnt-out vehicle was removed from the ditch.