Prime Minister Keir Starmer's decision last month to allow the British military to board ships from Russia's so-called "shadow fleet" has had no clear impact on the number passing through U.K. waters, a Reuters analysis shows.

In the month after Starmer's March 25 warning, at least 98 Russian vessels subject to U.K. sanctions transited its waters, roughly in line with each of the previous three months.

There has been no announcement of any boarding or detention of the ships, which typically have opaque ownership structures and can transport oil, grain and arms, often in support of Russia's war in Ukraine.

LSEG tracking data shows that 63 ships passed within 12 nautical miles of the coastline in the English Channel, the most direct route between the Baltic Sea and southern Europe. Another 35 travelled through Britain's Exclusive Economic Zone, which extends up to 200 nautical miles from the coast, mainly around northern Scotland.

"You have to quickly follow up with boardings, or those vessels will conclude that it was an empty threat — and that is the regrettable situation we are in now," said Elisabeth Braw, a maritime security expert at the Washington-based Atlantic Council.

Other European countries, including France, Belgium and Sweden, have in recent months boarded and detained Russian "shadow fleet" vessels. Britain's Ministry of Defence did not respond to a request for comment on its relative inaction.

Once a fabled sea power, Britain's navy is now the smallest it has been since the 17th century. At the same time, its military faces requests from allies to support operations in eastern Europe, the Arctic and the Middle East. London has 544 vessels linked to Russia's "shadow fleet" on its sanctions list.

The data suggests that at least 10 ships passing through had engaged in "spoofing" — turning off or manipulating their tracking systems — while crossing British waters. The Kremlin says sanctions against its ships are illegal and has called Britain's latest policy a "deeply hostile move" that could prompt retaliation.

Analysts say the U.K.'s lack of follow-through reflects several challenges, including the absence of a dedicated law enforcement coastguard — unlike France or Sweden — as well as the legal and economic complications of dealing with such a large number of vessels.