The government has identified a legal basis which it believes can be used to allow UK military to board and detain vessels in so-called shadow fleets, BBC News understands.

Russia, Iran and Venezuela have all been accused of operating ships without a valid national flag to avoid sanctions on oil.

Last week British armed forces assisted US troops in seizing the Marinera oil tanker, which American officials accused of carrying oil for Venezuela, Russia and Iran, breaking US sanctions.

To date, no UK military personnel have boarded any vessels, but officials have spent the last few weeks exploring what measures could be used.

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The Sanctions and Money Laundering Act from 2018 can be used to approve the use of military force, ministers believe.

It is understood there are plans for the armed forces to use these powers, in what is being described inside government as a ramping up of action against the ships.

It is not known exactly when the first UK military action might occur.

Two oil tankers subject to US sanctions were reportedly spotted sailing east through the English Channel towards Russia on Thursday.

The UK has already imposed sanctions on more than 500 alleged shadow vessels, which it believes are helping to fund hostile activity, including Russia's invasion of Ukraine.

Ministers say that action taken by the UK and its allies has forced around 200 ships off the seas, most of which will have been operating without a legitimate flag.

Ship flagging is the process used to register a vessel to a specific country, which then allows it to travel in international waters and offers it certain protections under law.

The government believes the new legal mechanism they have identified could be applied to any sanctioned vessels not legitimately flagged.

Officials say this would have included the Marinera tanker, which was seized last week.

The Marinera, a Venezuelan-linked ship previously known as the Bella 1, was stopped by the US Coast Guard as it travelled through the North Atlantic ocean between Iceland and Scotland.

The Ministry of Defence said the US asked the UK for assistance, and that RAF surveillance aircraft and a Royal Navy support ship RFA Tideforce took part in the operation.

Defence Secretary John Healey said the action was "in full compliance with international law", adding the UK "will not stand by as malign activity increases on the high seas".

Healey told MPs on Wednesday that the government was "stepping up action on the shadow fleet, developing further military options and strengthening co-ordination with allies".

It is understood that identifying this legal mechanism was one of the further military options that Healey was referring to.

The US has increased action against shadow fleet vessels, with five tankers seized in recent weeks.

A Ministry of Defence spokesperson said: "The defence secretary set out in parliament this week that deterring, disrupting and degrading the Russian shadow fleet is a priority for this government".

They added: "We will not comment on specific operational planning".

Transport Secretary Heidi Alexander said on Sunday that the number of insurance checks were being increased, with more than 600 ships stopped while sailing close to the British Isles.

Vessels not legitimately flagged generally have no insurance, which experts have warned could lead to a crisis if they were involved in an incident like an expensive oil spill.

But Alexander said it would not be appropriate to say how many alleged shadow vessels were known to have sailed in UK waters.

She told Sky News: "Providing you with that information only helps one person and that is President Putin."

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