There is an increasing link between drug trafficking and other crimes including terrorism, the European Union's drugs monitoring agency warned Tuesday in its annual report.
The warning comes in the aftermath of deadly suicide bombing attacks on Brussels last month, in which more than 30 people died. The fight against terrorism was already high on the EU's agenda, following attacks on Paris in November that claimed 130 lives.
Organized crime groups involved in the drug market are diversifying their activities, engaging in other forms of criminality including terrorism and forming cross-border alliances, the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) said in its report.
"Illicit drug production and trafficking remains one of the largest and most innovative criminal markets in Europe," said Rob Wainwright, the director of EU police agency Europol.
"As it grows more complex and becomes entwined with other forms of crime, and even terrorism, it represents a key threat to the internal security of the EU," he added.
Terrorist organizations are using involvement in the drug trade to fund their activities, although it seems not to be their main source of funding, the report said.
"It appears that many of those involved in these activities, often recently radicalised young people, may have a history of low-level criminality, including drug use or involvement in the drug market, and exploit their criminal links to conduct their terrorist activities in a range of ways," the authors wrote.
Efforts to combat terrorism should also include closer scrutiny of police data on "low-level criminals aspiring in the end to become terrorists," Wainwright said, while adding that the link was "indirect."
He also noted a degree of crossover between the drug trade and last year's migration surge into Europe, with more than 20 per cent of migrant smugglers also involved in drug trafficking activities.
Europeans spent an estimated minimum of 24 billion euros (27.3 billion dollars) on illicit drugs in 2015, the report said, making it one of the main profit-generating activities for organized criminals in Europe.
Cannabis remained the most widely used drug, accounting for 38 per cent of the market or more than 9.3 billion euros annually.
This was followed by heroin, responsible for a significant proportion of drug-related deaths; cocaine, Europe's most commonly used illicit stimulant; and synthetic stimulants such as amphetamine, methamphetamine and MDMA, the report said.