French customs officers believe the drugs, hidden among legitimate imports in a UK-registered lorry, were bound for the UK. They included a record haul of 400,000 'super-strength' ecstasy tablets.

Also seized from the truck were 200kg of cannabis resin, 220kg of herbal cannabis and 95kg of amphetamine sulphate, or "speed".

Dunkirk customs spokesman Rene-Claude Philippe said the haul appeared to reflect a sizeable increase in the flow of ecstasy from France to the UK.

He said: "We have noticed a great increase in the amount of ecstasy destined for Britain. In Dunkirk alone, we have seized more than 840,000 tablets this year - that is more than twice as many as were discovered in the whole of France last year."

The total consignment had a street value of 55m francs (£6.2m), he said.

'100 times stronger'

Tests on the ecstasy showed that it was extremely strong and would induce hallucinations in users, he said.

"It contains a molecule which is 100 times more intense than classic ecstasy."

A 59-year-old truck driver from London, arrested at the scene by customs officers, is now in custody in Lille, helping police with their enquiries.

He was driving on the N1 route from Belgium to Calais when his truck was stopped for a routine customs check on the Dunkirk ring-road at Teteghem, Mr Philippe said.

The drugs were found on three pallets hidden among a truck-load of legitimate goods, he said.

It is thought that the driver was planning to catch a cross-Channel ferry at Calais.