Tasking a minehunter with intercepting two Russian submarines and a support ship shows a "desperate shortage" of appropriate escort vessels, a former head of the Royal Navy has said.
Labour peer Admiral Lord West of Spithead also warned that the UK is moving into an "area of great danger" owing to a diminishing fleet size.
His comments came as the Sun newspaper first reported that Portsmouth-based HMS Cattistock was deployed to escort two Kilo-class Russian submarines and a Silva-class support tug through the English Channel last August.
Confirming the "glass-reinforced plastic" ship was tasked with the escort, a Royal Navy spokesman said it was the "most appropriate vessel for this particular task at that time".
"There is always one Royal Navy ship that is designated as the Fleet Ready Escort (FRE), although there are always a number of Royal Navy units available in UK waters that could conduct this role depending on the tasking," he added.
It is believed the two submarines and support ship were not operational at the time, were on delivery sailings, and that standby frigate HMS Westminster was busy undertaking another duty.
Pressed for his thoughts on the use of HMS Cattistock for the task, Lord West told the Press Association: "What it reflects is our desperate shortage of proper escorts.
"We were a great maritime nation, we still remain a maritime nation, but we are now moving into an area of great danger because we have such a lack of assets."
He said the use of the minehunter will have prompted the Russians to think that the Royal Navy "haven't got that much because they are having to do that".
"I am not worried that the Russian submarines were about to do something horrible to us, it is just that messaging," he added.
It is understood the Kremlin's three vessels were first being watched by Nato units as they passed through the North Sea, before HMS Cattistock took over escort duties.
Lord West said historically a frigate or destroyer - of which the Royal Navy has a total of 19 - would have been called upon for the task.
With just one ship usually available for the FRE he said this highlights how cut-back the Royal Navy has become, and stressed that by 1945 during the last big fight for the North Atlantic, the UK had 1,500 frigates and destroyers.
It comes amid recent warnings from the Ministry of Defence of an "upsurge" in Vladimir Putin's ships and vessels "transiting UK waters".
Earlier this month frigate HMS Westminster was scrambled to intercept two of Vladimir Putin's warships and two supporting vessels as they passed close to Britain.
HMS St Albans also spent Christmas Day escorting another Russian warship, the Admiral Gorshkov, through the North Sea.
Continuing the trend of growing concern towards Russia, Head of the Army General Sir Nick Carter used a speech last week to warn how the Kremlin boasts an "eye-watering" capability which Britain would struggle to match.