An off-duty police officer and paramedic were sprayed in the face with an unknown corrosive substance after stopping to help at the scene of a major car fire.

The pair joined other 'Good Samaritans' in lending a hand after a stolen car was involved in a serious collision with a lorry at around 7pm on Friday night.

The vehicle crashed into the a metal barrier at the side of the M11 near Chigwell and Loughton in Essex, before bursting into flames.

Just minutes before collision, police had been alerted to the blue VW Golf being stolen from outside a nearby shop.

The victim in the car theft, a man in his 20s, reported that one of the men held a knife in his hand.

An off-duty police officer and paramedic joined other 'Good Samaritans' in lending a hand after a stolen car was involved in a serious collision with a lorry at around 7pm on Friday night. It was there they were sprayed with a corrosive substance

No verbal or physical threats were issued, but upon seeing the knife, he ran from the scene.

The two suspects then drove away in the victim's car.

Less than 10 minutes later, the stolen car collided with the lorry on the M11, southbound, between junctions 5 and 4.

The car caught fire and after seeing the incident, passing drivers stopped to help.

Following the collision, the drivers of the stolen car left the vehicle and sprayed an unknown substance in the faces of two people who were among those who had stopped to assist.

One was an off duty paramedic, the other, an off duty police officer.

The suspects then stole another car - a white Vauxhall Astra - at the scene of the collision to make their getaway.

This car was later recovered in Dagenham, East London.

Police said medical treatment was received by the two people who were sprayed with the unknown substance, and it is not believed they will suffer from any long term effects.

Two ambulance crews, a rapid response vehicle and response car from Essex and Herts Air Ambulance were dispatched to the scene.

Both men sprayed with the substance were taken to hospital for further care.

Their conditions are not believed to be serious or life-threatening. The driver of the lorry was uninjured.

Kevin Brown, Director of Service Delivery at The East of England Ambulance Service NHS Trust (EEAST), said: 'This unprovoked attack on two Good Samaritans who were trying to help at the scene of a road traffic collision is deplorable.

'Sadly they were in the wrong place at the wrong time and we wish them a speedy recovery.'

The crash caused disruption to drivers, but the lane reopened later that evening following forensic recovery of the vehicles.

One suspect was described as Asian, and between 25 to 30 years old. He had a dark beard, and was wearing a baseball cap, a navy blue buttoned top, and dark coloured bottoms. He also had a small, black leather bag with him.

The other suspect was described as white, with short dark hair.

Both men sprayed with the substance were taken to hospital for further care

He was wearing a khaki coloured jumper and jogging bottoms and was thought to be between 20 and 25 years old.

Chief Superintendent Luke Collison of Essex Police, said: 'This was an shocking incident, and we have launched a significant investigation to track down the suspects and bring them to justice.

'On Friday night, good Samaritans who witnessed a serious collision stopped to provide urgent medical care, and were met by two dangerous offenders and seriously assaulted.

'Included in the group who stopped, were two off duty members of our emergency services.

'We are thankful that their injuries are not worse and that no other members of the public were seriously harmed.

Meanwhile a major operation has been launched in London to combat the rise in acid attacks, which have claimed multiple victims in the capital in recent months.

Five litre bottles of water are now to be stocked in emergency patrol cars across the the capital to provide vital and immediate treatment on the scene.

Water is one of the most effective ways of mitigating acid injuries and London fire brigade, which carries large volumes of water, will now respond with police to acid attack emergency calls to ensure that enough water is available to treat victims.

The capital's 999 response officers are also to be given protective equipment and trained in the appropriate medical response to treat injured victims.

Although police in east London are already equipped with acid response kits, under the new operation every car in London will be issued with water and protective equipment.

A trial scheme in east London, where attacks with corrosive liquids have been most prevalent, will see officers carry test kits which will allow them to determine what a liquid is during stop and search operations in the street.

These kits will allow police to safely investigate the contents of liquid in all containers, even in innocent-looking soft drink bottles.

Officials are now considering banning the sale of corrosive substances like certain household cleaners to under-18s.

Stricter regulation of other chemicals has also been mooted.

Anyone with any information should contact police on 101 or Crimestoppers anonymously on 0800 555 111.