A drug courier carrying an £8m heroin haul in a car led police on a 33-mile motorway pursuit.

Nathan Cook was jailed for nine and a half years on Tuesday after he was stopped by police and found to have 18.5kg of the drug in a high-purity form.

Traffic officers signalled Cook to pull over on a routine stop but he kept travelling at speed for 33 miles before he was boxed in by four police cars on a motorway.

It was then they discovered Cook, 28, was transporting high-purity heroin in a borrowed VW Polo.

Cook, of High Wycombe, Buckinghamshire, had earlier pled guilty to the offence on March 22, 2014, on the A74(M) Glasgow to Carlisle Road, when he appeared at the High Court in Edinburgh.

Advocate depute Andrew Brown QC told the court: "This case involves the discovery of a very significant amount of heroin after a routine stop by traffic officers policing the main west coast route from England to Scotland."

The prosecutor said Cook was acting as a courier delivering the drug for further cutting and distribution.

The court heard the haul was worth £832,500 at wholesale value but because of its high purity of 62% to 64%, could be adulterated and bulked out to make £7,999,300 worth of individual deals.

Officers decided to stop the northbound car and signalled it to halt with flashing lights and a siren but Cook refused to pull over and kept going from near Lockerbie, in Dumfriesshire, to Elvanfoot, in South Lanarkshire.

Mr Brown said: "Ultimately, after he had attempted unsuccessfully to leave the motorway at a number of junctions, four police cars successfully boxed in Polo and brought it safely to a halt."

Cook was brought out the car and placed on the roadway and handcuffed. Packages of heroin were recovered from the vehicle.

He had originally faced further charges including one of dangerous driving containing allegations that he had driven towards a police vehicle, forcing the driver to take evasive action and went onto the hard shoulder and undertook a van and trailer.

After he offered to plead guilty to the Class A drugs charge, the Crown accepted his not-guilty pleas to other charges.

Defence counsel Frances Connor said: "When he is eventually released from custody his intention is to gain proper, legitimate employment to support his young family. He just wants to provide for his family in a legitimate way."

Judge Lord Uist told him: "This was on any view a colossal quantity of a dangerous drug of very high purity and value.

"The public should be grateful to the police officers who decided to stop your car and so prevented this consignment of drugs getting on to the streets of Scotland.

"The damage that these drugs would have done to both individuals and the community as a whole had they got onto the streets is incalculable.

"You must have known the high risk you were taking when you undertook to transport these drugs and you must now face the reckoning, which has to be severe in this case."

Lord Uist said he would have jailed Cook for ten years but his sentence would be reduced to take account of periods he had already spent on remand.