Five men are in custody after a police raid at an Essex warehouse last night.
The Metropolitan police said today that a load from a lorry had been split and loaded on to two lorries, a van and a car at Askew Warehousing in Askew Farm Lane.
Two men fled the scene and are being sought.
Police said 12 pallets were being loaded, carrying what was believed to be 3.5 tonnes of cannabis resin, 1.5 tonnes of amphetamine, 0.5 tonnes of skunk and several kilos of mixed "cocktail" loads of as yet unidentified class A and B drugs.
Police estimate the minimum street value of the drugs at £20million.
Searches were later conducted at eight houses in Essex and London.
Nigel Eeles, of the Serious Organised Crime Agency (Soca), said: "We believe that we have caused a major disruption to a sophisticated organised criminal enterprise. The seizure is significant in size and trafficking of the drugs into the UK could have caused considerable harm to individuals and created substantial profit for this criminal organisation."
Five men aged 49, 67, 54, 70 and 69 were being held at Charing Cross Police Station.