Fears are rising that Russian shadow fleet crews could claim asylum if Britain seizes their vessels in the English Channel.
Both the Home and Foreign Secretaries are said to have raised worries that capturing tankers in British waters would enable Russian mercenaries to exploit immigration rules.
This issue appears to be hindering Sir Keir Starmer's plans to deploy special forces and the National Crime Agency against sanction-busting ships.
The Prime Minister pledged on March 26 to "go after" the shadow fleet.
But just days later, a shadow fleet tanker was escorted through the Channel by a Russian frigate, with three more sanctioned vessels following the next day.
The Royal Navy has not yet seized any ships, following legal advice from Lord Hermer that such action could breach "international law".
Whitehall sources have now indicated that sailors brought ashore by British forces would be entitled to seek asylum in the UK.
Many crew members on shadow fleet tankers are mercenaries employed by the Kremlin who could argue they face persecution if returned to Russia or their home countries.
PICTURED: The Russian tanker Marinera off the Scottish coast after being seized by American forces earlier this year
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Officials believe Russia may deliberately position spies aboard tankers, anticipating the vessels will be caught and brought to British shores.
Under asylum rules, foreign nationals arriving in Britain can claim protection if they face persecution due to the "political situation" in their home country.
Tory chief Kemi Badenoch told The Telegraph last night: "A legal system, defended by our lawyer-in-chief Keir Starmer, that allows asylum claims from Russian mercenaries is simply not fit for purpose.
"We need to put our national interest first and that means leaving the ECHR, because we know, having tried everything else, that nothing else will work to make our borders stronger."
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Officials believe Vladimir Putin may send spies on the tankers on purpose in the hopes they will go on to claim asylum in the UK
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Yvette Cooper and Shabana Mahmood are now working on a solution to the international law issue hamstringing the Navy.
The pair are said to be looking into plans which would allow sailors to seize shadow fleet tankers without allowing their crews to stay in Britain.
Officials are also examining how France has managed to impound tankers in the Mediterranean without accepting sailors as asylum seekers.
The Defence Secretary on Thursday admitted there was "more we can do" to halt the shadow fleet.
Defence Secretary John Healey admitted there was 'more we can do' to halt the shadow fleet
| PABut he argued that Russia's decision to escort tankers with warships demonstrated Britain had successfully diverted Vladimir Putin's resources away from Ukraine.
John Healey said the UK possessed "military options" for boarding vessels and was "ready to take action" to seize them.
Though Government sources say the Ministry of Defence would need to clear a high legal bar before launching any Channel raid, or risk facing action at an international maritime tribunal.
The MoD said: "We will not comment on specific operational planning or give a running commentary as this could compromise our ability to successfully take action against these ships, only benefitting our adversaries.
"In general terms, any target ship will be individually considered by law enforcement, military and energy market specialists before an operation is executed."