Sailing from the Gulf of Finland and through the Baltic Sea, the ageing and poorly-maintained tankers mainly transport crude oil as well as refined products such as gasoline and diesel.
Tankers from Russia's "shadow fleet" circumventing European sanctions navigated through Denmark's waters at a rate of almost one a day in 2025, data compiled by the Danish Maritime Authority showed on Wednesday.
"In 2025, there were 292 voyages with EU sanction-designated tankers in Danish waters," the authority said in an email to the AFP news agency.
Russia has reportedly built up a flotilla of old oil tankers of opaque ownership to get around sanctions imposed by the European Union, as well as the United States and the G7 group of nations, over Moscow's 2022 full-scale invasion of Ukraine.
The sanctions, aimed at limiting Moscow's revenues used to pursue its war, have shut out many tankers carrying Russian oil from Western insurance and shipping systems.
Sailing from the Gulf of Finland, through the Baltic Sea and into Danish waters, the ageing and poorly-maintained "shadow fleet" tankers mainly transport crude oil as well as refined products such as gasoline and diesel, raising concerns about the risk of an oil spill.
The size and movements of the "shadow fleet" have been hard to pin down, but thanks to the EU sanctions list, Danish authorities were able to monitor and register the ships turning up in its waters last year.
The EU lists 598 vessels that are banned from European ports and maritime services.
"The Danish authorities are closely monitoring ships in Danish waters" and "also cooperate closely with like-minded countries in the Baltic Sea region," the Maritime Authority said.
"In addition, a number of concrete measures have been taken to strengthen maritime safety and the protection of the marine environment and seafarers."
Some experts and political leaders also suspect the vessels of carrying out sabotage, as part of a "hybrid war" by Russia against Western countries.
In Finland, three crew members from the Cook Islands-registered "shadow fleet" ship Eagle S were accused of dragging the vessel's anchor across the seabed in the Gulf of Finland in December 2024, damaging five undersea cables.
A Helsinki court dismissed the case, saying it was beyond its jurisdiction.
In late January, the French navy boarded a sanctioned vessel believed to be a Russian oil tanker flying a false flag in the Mediterranean.