Britain’s Royal Navy tracked and followed a Russian frigate every day last month as it sailed from the Atlantic to the North Sea as Moscow steps up its maritime presence after UK threats to seize shadow fleet oil tankers.

The Russian navy’s Admiral Grigorovich escorted six Russia-linked vessels during April, including at least three under economic sanction passing east through the Dover strait, while being watched continuously by four UK ships and helicopters.

The frigate, which naval spotters believe remains in the North Sea, was able to maintain its presence near Britain by taking on supplies near Galloper windfarm off the Suffolk coast.

A second frigate, Admiral Kasatonov, passed through the Channel towards the end of April escorting two merchant ships believed to be heading towards Tartus in Syria, one of whom, the Sparta, is associated with the transport of arms.

That convoy was monitored by a British auxiliary ship, the RFA Tideforce, part of a wider commitment of the UK’s stretched naval resources as maritime tensions between London and Moscow increase.

John Healey, the defence secretary, said a month ago that the British navy had followed three Russian submarines on a monthlong mission sailing over pipelines and cables near UK waters, probably spying on the infrastructure below.

Elizabeth Braw, a security expert with the Atlantic Council thinktank, said: “This is completely disproportionate; navies normally only escort vessels when there is a clear military threat, such as from the Houthis in the Red Sea.”

As well as the occasional transport of arms, Russia relies on the “shadow fleet” of often old, poorly maintained tankers, sailing under third country flags of convenience for roughly half its seaborne oil exports.

Braw added: “Russia has decided that this is an indispensable source of income that could be disrupted. Clearly, if the Russians thought these vessels were not going to encounter problems, they would not allocate a frigate.”

Keir Starmer released a statement on 25 March, announcing the military was able to seize ships subject to existing economic sanctions that were transiting through UK waters.

This appeared to announce powers to seize vessels, deemed to be in breach of maritime law, that the UK already had – but it also confirmed that a cross-government process had been agreed by which potential seizures would be evaulated.

The prime minister said the UK would be going after Vladimir Putin’s “shadow fleet even harder” to help safeguard the UK and to starve “Putin’s war machine of the dirty profits that fund his barbaric campaign in Ukraine”.

Since then the UK has not led the seizure of any shadow fleet tankers and is likely to have been deterred by the presence of Russian warships in close proximity.

Braw said: “The Russian government has shown it is willing to use its naval power to protect vessels that break maritime rules. It makes it much more difficult for coastal states to intervene. You have to be willing to risk a confrontation with the escort.”

Other European countries have seized shadow fleet vessels this year, though it is not clear if any were escorted. Sweden has detained five tankers, the last of which, the Jin Hui, was boarded on Sunday on suspicion of flying under a false flag.

France has seized two Russia-linked tankers with UK assistance this year, though each was released after the owners paid a fine. Belgian special forces seized an oil tanker bound for Russia with French help at the end of February; prosecutors said it was falsely flying the flag of Guinea.

The US seized the Bella 1 or Marinera tanker in early January, which was reportedly escorted by a Russian submarine. It had been sailing towards Venezuela in defiance of a US blockade before turning around to evade seizure. In the days that followed its crew painted a Russian flag on the side but it was eventually run down and captured in the north Atlantic by the US with help from the UK.