Police investigating what they say is the UK's biggest ever drugs conspiracy have arrested 13 men in dawn raids.The arrests follow a National Crime Agency investigation tracking drugs allegedly smuggled into the UK in lorry loads of vegetables and juice.The conspiracy was around the importation of more than 50 tonnes of drugs, worth millions of pounds, from the Netherlands, officers say.NCA branch commander Jayne Lloyd called the arrests "massively significant".The arrests took place in London, Manchester, Stockport, St Helens, Warrington, Bolton, Dewsbury, and Leeds."We believe it's the biggest ever conspiracy that we've seen in the UK," said Ms Lloyd, NCA regional head of investigations for the north of England.The 13 are suspected of being part of the UK arm of a well established organised crime group that allegedly used Dutch and British front companies to import heroin, cocaine and cannabis.Four men and two women arrested in April by Dutch police, on a European Arrest Warrant, are awaiting extradition to the UK and are suspected of being part of the same conspiracy.The 13 men are alleged to have imported drugs on numerous occasions between February 2017 and October 2018, including three consignments intercepted in 2018 with a total street value of more than £38m."We suspect these men were involved in an industrial-scale operation - the biggest ever uncovered in the UK - bringing in tonnes of deadly drugs that were distributed to crime groups throughout the country," Ms Lloyd said. The 13 arrested are believed to be part of a drugs smuggling gangThe investigation is linked to an operation in 2015 that saw 13 people jailed for a total of 176 years after the seizure of more than 100kg of heroin.Ms Lloyd said she hoped the arrests would "have a big impact" on the county lines drugs trade - in which drugs are transported form cities to users across the country - which was having a "devastating effect on the public, vulnerable children and the economy".The NCA says the investigation involved European police forces and agencies, including Europol and Eurojust, as well as the UK Border Force and Revenue & Customs."By working closely with partners here and overseas, in particular the Dutch national police, we believe we have dismantled a well established drug supply route," Ms Lloyd said.