Russia appears prepared to create “environmental havoc” by sailing unseaworthy oil tankers through the Baltic Sea in breach of all maritime rules, the Swedish foreign minister has said.

Speaking to the Guardian during his first visit to London since Sweden became a Nato member, Tobias Billström called for new rules and enforcement mechanisms to prevent the ageing and uninsured Russian shadow fleet causing an environmental catastrophe. About half of all Russian oil transported by sea passes through the Baltic Sea and Danish waters, often operating under opaque ownership, and using international waters to try to avoid scrutiny.

The fleet generates a huge amount of revenue for Russia’s war machine, bypassing western sanctions that try to block access to insurance if Russia sells the oil above $60 a barrel. In practice as little as 20% of Russian oil is sold below the price cap.