CUSTOMS officers investigating a North-East fuel tax fraud uncovered an amazing trail that led to a Real IRA cell conducting a mainland bombing campaign, it was revealed last night.

The real story behind the breakthrough that led police to the terrorists responsible for bombings in London and Birmingham has never been told - until now.

When the five terrorists went on trial at the Old Bailey in April, details of the operation were kept secret because two North-East men and a businessman from Scunthorpe were awaiting trial for evasion of excise duty.

Now the case has been heard, the full details have finally emerged.

Customs spent months investigating large-scale fuel movements between Chilton, in County Durham, and a farm at West Ardsley, near Leeds.

They suspected the tanker-loads of red diesel - duty free fuel supplied for industrial use only - were being treated to remove the colour and sold on the black market as genuine diesel.

But when they raided the West Yorkshire farm, they discovered that two of the men involved - Irish brothers Robert and Aiden Hulme - had links to the terror cell being sought for bombings in London and Birmingham.

The breakthrough allowed police to arrest the brothers and round up the rest of the bombers.

Customs officers stressed last night that they had no proof that profits from the fuel fraud had funded the terrorists.

However, the Old Bailey heard earlier this year that the Real IRA cell had funded its activities with profits from illegal diesel sales and that the West Yorkshire farm, plus another nearby, had been the base for their operations.

Delighted Customs chiefs last night hailed the operation as a tremendous success.

A spokesman said: "This operation was the most sophisticated of its kind ever seen by customs officials.

"We believe it netted the gang behind it profits of up to £2.5m."

The fuel fraud inquiry, codenamed Operation Warren, was launched by Customs officers in August 2001.

Between March and November of that year, 47-year-old Martin Simpson, trading as Copleys Fuels, delivered about 4.3 million litres of red diesel to haulier Robert Surtees at Standalone Farm, Chilton, County Durham.

Surtees, 33, who lived in Newton Aycliffe and was described by Customs as the gang's ringleader, had the fuel transferred into a tank, which was concealed in a blue tipper trailer.

This was then picked up by Peter Brown, from Hartlepool, who delivered it to Manor House Farm, West Ardsley. He made at least one delivery every working day.

The fuel would then be transferred by Brown and Allan Aggett, from Ashby, Scunthorpe, into a 30,000 litre tank.

The diesel would then be "washed" with chemicals to remove the red dye, leaving the end product so closely resembling Derv that it could only be detected by experts.

The Derv, which attracts a much higher rate of duty than red diesel, was then sold to retailers all over the country at one or two pence below the market price.

After mounting a covert operation, Customs officers carried out a dawn raid on the West Ardsley farm on November 14, 2001, where Brown and Aggett were arrested. Surtees and Mr Simpson were arrested in County Durham the same day.

Earlier this year, Surtees whose last known address is Badminton Grove, Woodham, County Durham, Brown, 53, of Wynyard Road, Hartlepool, and Aggett pleaded guilty to the fraudulent evasion of excise duty. They are due to be sentenced at Bradford Crown Court on Monday for the offence, which carries a maximum sentence of seven years in prison.

The charges against Mr Simpson, of Tilleys Row, Butterknowle, County Durham, will remain on file.

Customs accepted his claim that he had no knowledge of how the red diesel was being used. He has offered to pay the £15,000 profit made from his dealings with Surtees to the Treasury.

During the operation in West Yorkshire, Customs discovered a link to nearby Hill Top Farm, which was subsequently found to be the terror cell hideout.

When police raided the second farm, they found a Vauxhall Cavalier that contained a pistol, electronic detonators, a hand grenade and timing unit. Intelligence experts believe it was being prepared for another car bomb attack.

Robert and Aiden Hulme, who worked at the diesel laundering plant, were part of a Real IRA cell responsible for a car bombing campaign in London and Birmingham.

The brothers, together with Noel McGuire, James McCormack and John Hannan, were convicted of terrorist offences at the Old Bailey in April.

The gang was accused of plotting to plant a device outside the BBC television centre in London in March 2001.

A second car bomb detonated in Ealing Broadway, west London, in August of that year, and a third in Smallbrook, Birmingham, in November.