Tankers of Russian oil sailing through the Danish straits will be more likely to crash and spill their cargoes if they transit the shallow treacherous waters without the specialist pilots usually provided to vessels in the channel, Denmark’s maritime authority has warned.

The narrow stretch of water between Denmark and Sweden at the mouth of the Baltic Sea is a key trade route for Russian oil heading by sea to markets around the world.

Under a 165-year-old treaty signed in Copenhagen in 1857 all international vessels have the right to transit the straits but Denmark must make pilots available to help vessels navigate its many islands and sandbanks.

The UN’s International Maritime Organisation “highly recommends” the use of pilots but it is not compulsory. Members of the Danish shipping industry fear that sanctions on Russian trade could lead to a rise in dangerous unpiloted vessels.

“Failure to comply with the rules and recommendations of the IMO will not only pose an environmental risk to Danish territorial waters. It will also pose a risk to the safety of navigation and the crew members on board the ships,” the Danish Maritime Authority said in response to questions.

“We therefore urge the global shipping sector to continue to adhere to all the rules and recommendations of the IMO,” it added.

Russian oil exports have dropped only marginally since western restrictions were introduced earlier this year in response to the invasion of Ukraine. Roughly 1.5mn barrels a day of crude oil — approximately 1.5 per cent of global supply — continue to pass through the Danish straits on their way to the North Sea and the Atlantic Ocean, according to data from the commodity analytics group Kpler.

So far most vessels have continued to follow the IMO recommendation. In August, 196 oil tankers passed through the main channel in the Danish straits, known as the Great Belt. Over 95 per cent used a pilot, Danish Maritime Authority data show. That compares with 92 per cent in August 2021, when 129 oil tankers passed through the channel.

In March, Danish pilots called for the service to Russian vessels to cease. However, the Danish authorities have stated that it must continue. “It is a matter of safety of navigation and prevention of environmental disasters in Danish territorial waters,” the Danish Maritime Authority told the Financial Times.

Some in the industry are concerned that when EU sanctions on the seaborne trade of Russian oil come into full effect from 5 December it will complicate or even prevent Denmark from providing pilots to tankers carrying Russian cargoes. Under the sanctions, the provision of maritime services to such vessels will be banned, although it is possible that pilotage could still be permitted for the transport of oil to “third countries” outside the EU.

Viktor Katona, an oil markets expert at Kpler, said he thought a solution would be found. “The roughly 1.5mn b/d of crude alone passing through the straits is a sizeable part of global demand. If something happens to it, prices spiral again . . . No one would be happy about it.”

The provision of pilotage services to vessels carrying Russian cargoes through Turkey’s Bosphorus Strait had also continued despite tightening global restrictions, he added.

However, even if pilotage services remain available, vessels carrying Russian crude cargoes could choose not to use them, particularly if more of that trade moves on to older “dark vessels” in order to circumvent restrictions, as has happened with Iranian and Venezuelan oil.

Maria Skipper Schwenn, executive director for climate, environment and security at Danish Shipping, a trade organisation, said it was paramount that pilotage services continued.

“As a significant shipping nation with more than 7,000km of coastline, we are obliged to prioritise safe and environmentally sound navigation,” she said.