The cocaine was worth about �30m

James Mair, 38, David Frew, 55, and Sean McAdam, 26, all from Ayrshire, along with Gavin Grant, 38, from Lanarkshire, were all found guilty of importing 500 kilogrammes of cocaine.

The High Court in Glasgow heard the drugs, worth �30m at street value, were concealed in three container loads of bales of raw rubber.

Customs officers followed the cargo from where it had landed in Felixstowe to Scotland.

After a massive surveillance operation the gang were arrested at industrial estates in Glasgow and Kilwinning, Ayrshire.

Five hundred kilos of cocaine wasn't just for the Scottish market BBC Scotland's Bob Wylie

Sentences will be handed out in three weeks time.

The court heard Mair, who owned a small goods delivery business, allowed his rented premises in the Buchanan Business Park at Stepps, Glasgow, to be used as a front for the operation.

Documents showed that the rubber imported from Panama was ordered in his company's name.

Customs investigators became deeply suspicious as it is virtually unheard of for raw rubber to be exported to the UK from Central America.

A fifth man, McAdam's brother, William, 40, of Abbots View, Kilwinning, was acquitted with a unanimous not guilty verdict.

A sixth Scot, from Kilmarnock, who helped mastermind the operation and goes under three different names, is on the run after a Scots warrant and an Interpol "red notice" was issued for his arrest.

Cocaine removed

The five convicted Scots smugglers now face heavy sentences estimated to range from 15 years to even life imprisonment.

They will learn their fate when the trial judge, Lord Carloway, sentences them at the High Court in Edinburgh on 6 June.

The customs investigation, named Operation Thyme, began on 4 September last year as soon as the three suspect containers were unloaded in Felixstowe.

Each of the rubber bales was then put through a portable X-ray scanner which detected secret compartments with square objects inside 125 of them.

Working for two days, customs officers cut open each bale, taking out the contents and substituting them with sand, before sealing each bale again.

'Muscle and money'

Once the container doors were closed up with new bolts and rivets painted with a special chemical to disguise their newness they were sent on their way by sea to the Port of Grangemouth.

They then travelled to Mair's premises in Stepps, monitored all the way by investigators.

Surveillance continued for a week before customs investigators swooped and made arrests.

BBC Scotland's investigative reporter Bob Wylie believes the case is a worrying development.

"Five hundred kilos of cocaine wasn't just for the Scottish market, so the indications in this case are that Scottish criminals have got muscle and money and that they were prepared to bring in this amount of cocaine which they would have dealt out throughout England and Wales and Scotland," he said.