Police have seized drugs worth an estimated £2m and arrested nine people as part of a coordinated operation in Northern Ireland and northern England.
Police said they believe they have recovered drugs worth £1m during searches in Northern Ireland while a further £1m was seized in England.
Six kilos of cocaine, 22 kilos of amphetamines and 20 kilos of cannabis were found in Kilkeel, County Down.
Officers arrested five men in NI and three men and a woman in England.
The Police Service of Northern Ireland (PSNI) led the year-long operation which also involved Greater Manchester Police, Lancashire Police and the Serious Organised Crime Agency (SOCA).
Det Supt Esmond Adair, the officer in charge of the operation, said it was "probably the most significant drugs seizure" that has taken place in Northern Ireland "in recent times".
Referring to the valuation of the drugs he said: "Those are conservative estimates - you could triple and quadruple that when you get down to street value."
The arrests and seizures were made on Saturday and during a follow-up search in the Belfast area, six kilos of amphetamines and £100,000 in sterling and euro notes were also seized.
As part of the same operation, one kilo of cocaine, 22 kilos of amphetamines and 20 kilos of herbal cannabis were recovered by police from a house in Preston, Lancashire.
Eight searches were carried out in Northern Ireland, resulting in the arrests of five men.
The operation centred on Kilkeel, but there were also searches in Belfast, Lisburn, Hillsborough, and Lurgan.
There were five searches in England and three men - who are being taken to Northern Ireland for questioning - and one woman were arrested.
Seven of the arrested men are from Northern Ireland, the woman is from England and the ninth man is a Dutch national.
Det Supt Adair said police believed the drugs had been taken from Holland to England and on then to Northern Ireland.
"This has been a successful overt stage of a long-term proactive investigation involving the PSNI and partner agencies against an organised crime gang which we believe was importing drugs into Northern Ireland," he added.
"These drugs obviously hit our communities they cause untold harm to our communities.
"It is highly important that we take this type of action and prevent these drugs from getting on the street.
"The high quality purity of these drugs is very significant.
"These drugs would be cut up and we would have significant quantities going to street level so this is a very significant seizure."