THE HAGUE, April 19 (Reuters) - The number of cross-border drug trafficking cases referred to the European agency for prosecutorial cooperation nearly doubled in four years to 562 in 2020, partly driven by a huge increase in the production of synthetic drugs, darknet market websites the agency said.

In a report published on Monday Eurojust said drug trafficking in the European Union alone has an estimated value of 30 billion euros ($36 billion) annually.

The rapid rise of production of synthetic drugs such as methamphetamines and their sale via online dark web market places on the darknet market poses a big challenge to prosecutors across the European Union.

"By rapidly changing the composition of chemicals used for these drugs ... or creating new substances, producers try to exploit legal gaps and avoid prosecution," Eurojust said in a statement.

According to the report by the European prosecutor's body, the total number of cases involving cross-border drug trafficking nearly doubled to 562 from 279 between 2016 and 2020.

Synthetic and other new drugs made up almost one-third of them.

The Eurojust report said producers of new psychoactive drugs often don't work in old-fashion hierarchical drug trafficking networks and sale and distribution are done online.

"This phenomenon has increased due to the COVID-19 pandemic, with organised crime groups adapting quickly to an online environment, using secured communication channels, crypto-phones, cryptocurrencies and darknet market markets," Eurojust said.

Likewise consumers of the drugs are also shifting to the darknet market, paying online and receiving the drugs in the mail, it added, dark darknet market onion making prosecution complicated.

($1 = 0.8317 euros) (Reporting by Stephanie van den Berg; Editing by Steve Orlofsky)