John McHugh, 54, Joseph Boland, 54, both from Busby, along with Martin Gray, 41, from Ardrossan, and Garry Ralph, 38, from Glasgow, were all arrested following the raid.

The four pleaded guilty to being involved in the supply of cannabis resin at the High Court in Glasgow on 2 August this year. They were remanded to custody and will be sentenced later this month.

A statement by the Scottish Police Services Authority said it is believed that the group, headed by McHugh, imported the drugs from mainland Europe for onward supply in Scotland.

"During an intensive surveillance operation, on the morning of 24 July, officers witnessed the drugs, which were wrapped in 34 individuals bales, being delivered in a van to business premises at Scotts Road in Paisley, which at the time were being operated as The Print Finishers. 

"McHugh and Boland were observed unloading the drugs from the van. Subsequent inquiries revealed that the vehicle was associated with a company operated by McHugh."

On the same day, Gray and Ralph were arrested after being observed taking possession of quantities of the drugs from the premises in Paisley, near Glasgow.

Officers then detained McHugh and Boland at the industrial unit where they were found with just under 800 kilograms of cannabis resin, valued at £2.4m.

Allan Moffat, SCDEA detective chief superintendent and head of the investigations group, said: "These men were intending to supply significant amounts of cannabis to parts of Scotland and the seizures made during this investigation provide a clear indication of the level of profit these individuals would have generated from their criminal activity."