IT started with a loud roaring noise that shattered the early morning peace.

The deafening din signalled the beginning of a major incident that resulted in vast amounts of thick black smoke billowing from Fawley refinery.

Furious residents rang environmental health officials to complain about the huge cloud and nauseating smell.

Yesterday, more than 24 hours after the drama began, smoke was still pouring from two of the flare stacks that burn off excess gasses.

A joint investigation has been launched by the Environment Agency and the Health and Safety Executive. Experts are examining the cause and consequences of the incident, which began with a fire in the refinery's steam plant.

The blaze wrecked the equipment that enables staff to inject steam into the flares and thus dilute the emissions.

However, refinery owner Esso says air samples taken in the area have confirmed that the smoke does not pose a danger to people's health.

An Environment Agency spokesman said: "We're the authority that regulates Fawley and will be checking to ensure they complied with all the regulations.

"We'll be looking at why the fire started, what the consequences were and whether there are any lessons to be learned."

The spokesman said the smoke was caused by a safety procedure that aimed to dispose of excess gases by burning them at high temperatures.

He added: "The smoke does not pose a significant risk to public health or the environment."

The Health and Safety Executive said it was receiving regular updates from the agency and would be visiting the refinery next week to make its own inquiries. The blaze broke out last Wednesday at 7.30am and was extinguished by the refinery's own fire brigade.

Fawley resident Mike Cooper said: "I heard a huge roaring sound - and then all the stacks starting flaring. Smoke was still billowing out of the refinery when we came back from the New Forest Show at 4pm."

Eastleigh Borough Council has received several complaints, all from people living in the Netley area.

Netley resident John Stranger said: "Smoke was going all over the place on Wednesday.

"Yesterday it was still pouring out of two or three stacks and appeared to be heading up the Hamble River in the direction of Botley and Hedge End.

"It's not unusual to see black smoke emerging from some of the stacks but it normally only lasts for four or five minutes.

"This has been going on for 24 hours and must be having an effect on people's health."