Investigators are trying to discover how yesterday's fire at Tilbury power station broke out.

The station owners, RWE npower, returned to the station this morning. They are conducting a full internal investigation.

RWE said: "Units nine and 10, where the fire occurred, are being emptied of fuel. Unit eight is currently generating energy in order to safely empty its hopper of any remaining biomass.

"We will be reviewing the extent of the damage to the station over the coming days."

Through the night 12 crews worked together inside the unlit building to keep pumping foam onto the wood chip biofuel which had caught light at breakfast-time yesterday.

Around midnight work began to unload one of the two fuel hoppers which had caught alight.

An Essex fire brigade spokesman said: "This operation will be carried out under carefully controlled conditions. It is anticipated the operation to remove embers from the two hoppers involved in the fire will take around two days."

There is a danger the fuel could reignite so six pumps are being used in short shifts to keep pumping foam onto the fuel.

The first sign of a problem came yesterday morning.

Thick black smoke could be seen from Gravesend down to Higham and beyond after a fire started at Tilbury Power Station, on the opposite side of the River Thames.

Speaking about the crews fighting the intense blaze, chief fire officer David Johnson said: "They are doing an absolutely fantastic job in dangerous and punishing conditions. Inside the building it's hot, full of smoke and extremely dusty.

"We expect the fire to be out within the next two to three hours if everything goes according to plan."

At the height of the fire on Monday morning, more than 100 firefighters were involved in the operation.

Flames were coming from high up in the main building - a wood pellet hopper containing 2,100 tonnes of machinery - and warning sirens could be heard.

The fire broke out in a fuel storage area about 7.45am and around 100 firefighters were quickly on the scene, according to Essex Police.

The whole building was heavily smoke-logged and firefighters were trying to assess how to tackle the blaze.

Chief fire officer Johnson, in charge of the incident, said initially: "The safety of our crews is paramount. Because of the position of the fire and the structure of the building it is not safe to commit them to the building immediately. We are getting as close as we can.

"The fire involves some 4,000 tonnes of fuel in storage cells."

Several high voltage power cables were isolated by the fire service who using three aerial ladder platforms among their 17 engines at the scene.

Less than an hour after the fire broke out, a second fire started in a pile of wood close to berths 27 and 28 at Tilbury Docks. That was soon dealt with.

Chief fire officer David Johnson said it was one of the most challenging fires he had had to deal with in his 20-year career because of the technical complexities.

It involved up to 6,000 tonnes of material high in the roof of the power station.

The number of firefighters rose to 120 firefighters by late morning. They used helicopters to survey the building and identify hot spots, while inside thousands of gallons of foam was pumped onto the burning fuel to try to put it out.

The source of the fire is material used to drive the power station. It is imported from Europe by ship. The same material caught fire aboard a ship a few weeks ago, sparking another major emergency for the Essex fire service.

Early in the fire, Mr Johnson said: "We expect this to be a protracted incident going on for some hours, if not days."

The power station was closed down late on Monday morning at the height of the fire.