The 18-year-old, who asked not to be named, inadvertently spilled paint thinner on his trousers as he worked in a depot shed spraypainting a lighting tower.

As he walked to find a change of clothes in a locker a gas burner heater ignited his trousers. He managed to run outside where a quick-thinking colleague hosed him down.

The worker suffered burns to both legs, his left arm and his hand and had to be treated in hospital for 16 days. Since then the young man has undergone skin grafts and has been unable to work for six months.

An investigation by the Health and Safety Executive after the fire incident in October 2009 found the spray shed had several health and safety failings.

O’Keefe Construction (Greenwich) pleaded guilty to breaching sections of the Health and Safety at Work Act at Sevenoaks Magistrates’ Court and was fined £20,000. It was ordered to pay costs of £6,329.

The HSE said that paint containers should have had their lids on and been stored in fire-resistant boxes.

The company should also have been aware that the mixing of paint and thinners in the shed would lead to a potentially explosive atmosphere, so a gas burner with an open flame should not have been used in the same space.

HSE investigators found O’Keefe had executed a risk assessment, but had not implemented the measures identified.

A further management action plan, dated three years after the original assessment, re-iterated these measures, but they had still not been put in place at the time of the incident.

Caroline Penwill, HSE Inspector, said: “The process of risk management involves assessing the risks that arise in the workplace and putting sensible health and safety measures in place to control them.

“In this case, the company had assessed the risks from paint spraying and had identified measures to control the risks, but had not put them in place. It is important that the findings of a risk assessment are acted upon. Had the company done so, then this terrible incident could have been prevented.”