Fears have been raised the UK's defences are at risk due to a 'chronic' shortage of 'proper' Royal Navy warships available to shadow Vladimir Putin's naval armada.
Warships, submarines and spy vessels from the Russian leader's fleet have been seen skulking off the coast in the Channel, North Sea and Irish Sea in recent months.
Britain's navy has repeatedly scrambled warships and submarine-hunting helicopters to escort the vessels, in operations defence officials insist are 'routine'.
But increasingly, the navy appears to be relying more on lightly-armed patrol ships instead of its fleet of more powerful frigates and destroyers to carry out the task.
Only half of the nation's fleet of ageing Type 23 frigates - which specialise in seeking out Putin's underwater nuclear boats - are understood to be available.
While just one of the navy's six £1billion-a-piece Type 45 destroyers is currently free to respond to Putin's armada in UK waters, with four in refit or maintenance.
Defence experts fear the increased reliance of smaller offshore patrol vessels (OPVs) to carry out the task hints at how stretched the navy has become.
Commander Tom Sharpe, who previously skippered an anti-submarine frigate, was deeply concerned about the use of OPVs to shadow Putin's subs.
'It should be a "proper" warship that does the escorting for reasons of presentation, deterrent and intelligence gathering - and speed - but we pretend it doesn't because we don't have enough ships,' Cdr Sharpe said.
The operations were part of a wider deterrent effort by the armed forces to show the Russian tyrant 'we can see you, and we can match you if needed', he added.
But speaking to Daily Mail, the veteran naval commander warned: 'But for it to work, you do actually have to be able to match them, and patrol vessels, as useful as they are, can not.
'And it's not just optics; there are elements of the intelligence gathering part that a frigate is better at too. If you chose to send a patrol vessel to do it then that's one thing. But if you send a patrol vessel because you have to - because you don't have enough frigates - then you are not deterring, you are actually encouraging.'
In recent months patrol ships, including HMS Mersey, HMS Tyne and HMS Trent, have been used to chase down submarines or warships.
In May HMS Tyne shadowed Russian submarine RFS Krasnodar solo and corvette RFS Boikiy during two separate transits through the English Channel.
In June, HMS Trent was tasked to track Russian frigate Admiral Grigorovich, which is armed with a powerful 100mm naval gun and cruise missiles.
While last month, Portsmouth-based patrol ship HMS Mersey was called to action alongside a Wildcat helicopter from 815 Naval Air Squadron and a specialist submarine hunting Merlin aircraft from 824 Naval Air Squadron to follow RFN Novorossiysk as the Russian sub sailed west through UK waters.
The ships have also backed up frigates and destroyers on similar missions this year, when those were vessels sent out to hunt Putin's ships down and monitor them.
An observer from 815 Naval Air Squadron tracks the Krasnodar from a Wildcat helicopter from the UK Carrier Strike Group.
This close-up of the helicopter observer's screen shows the Krasnodar in detail as the Royal Navy keep close track of it in the Atlantic, before it entered the Channel.
Commodore Steve Prest was worried about the situation. Although downplaying the use of OPVs to shadow the Russians, he raised concerns about the overall state of the navy's fleet.
'We're not expecting these ships to fight or have a scrap with a bunch of Russians in the Channel,' he told Daily Mail.
'It's about sending an OPV to keep an eye on them to let know we're there and that should anything kick-off, we can call the cavalry.
'[But] the question then is, is there a cavalry to call if you need to escalate?'
The current size of the fleet has shrunk in recent years after a number of the navy's 13 ageing Type 23 frigates were decommissioned.
The remaining eight that are left have already had extensions to their lifespans to keep them in the fleet longer, with many set to be retired in the next few years.
The Type 23s have been the backbone of the navy's anti-submarine fleet since the 1980s when they came into service. They were originally designed with an 18-year life span.
They are due to be replaced by the advanced Type 26 frigates and the more basic Type 32s. However, these programmes have been plagued by delays.
Despite the setbacks, the Ministry of Defence (MoD) remains confident there will be no capability gap as older Type 23s are phased out and replaced.
However, defence sources are not as optimistic. Behind closed doors in Westminster, there are deep-seated concerns such a feat will not be achievable, insiders warn.
'The problem is the navy has so few surface escorts that are serviceable... and they are increasingly using OPVs to fill in the gaps,' one defence source said.
Defence sources say there has been an increase in the number of Russian vessels sailing past the UK.
Currently, Daily Mail understands the navy has four frigates available in UK waters, and one state-of-the-art £1billion Type 45 destroyer.
All the other ships in the 14-strong frigate and destroyer fleet are either in refit or maintenance, while three - HMS Lancaster, HMS Richmond and HMS Dauntless - are deployed on operations in the Middle East or in the Indo-Pacific with the carrier strike group.
Cdre Prest added having four of the eight frigates available was good, given normal ship rotations, which traditionally would see one third deployed, with another third in training while the remainder is in refit or undergoing routine maintenance.
But he warned: ‘Of the fleet we’ve got, it’s not too bad. The question is, is the fleet big enough?
‘The people in the Royal Navy are doing a good job in keeping ships available that are ageing and limited in number but we have an old fleet of frigates that need to be replaced.
‘The Type 26s and Type 32s need to get a move on and be brought in.’
Shadow Armed Forces Minister, Mark Francois, feared the Senior Service was on the cusp of a major shortage of prime surface fleet warships.
It comes amid a 'perfect storm' which has seen much of the Type 45 destroyer fleet hauled in for lengthy refits to improve their flawed power and propulsion systems.
Meanwhile, issues in the construction of the Type 26s at BAE Systems' shipyard in Glasgow, Scotland, have caused a costly 12-month delay to the programme.
The first ship in the fleet, HMS Glasgow, was meant to reach initial operating capability in October 2027.
However, earlier this year, defence procurement minister Maria Eagles warned this was more likely to be October 2028 - at a cost of £233million to the multi-billion pound programme.
Daily Mail understands one of the issues saw HMS Glasgow being hit by a major setback with its David Brown gearbox, which was delivered late and was not up to scratch.
Sources said it led to engineers cutting a hole in the ship's hull to carry out the required work to modify it. The MoD was reportedly not told about the gearbox failure until 20 months after the issue was identified.
Meanwhile, there are concerns the navy will not deploy another frigate to the Gulf after HMS Lancaster returns from her task of protecting the region's major shipping lanes, in a long-standing mission dubbed 'Operation Kipion'.
Mark Francois, shadow armed forces minister, told the Daily Mail: 'Assuming HMS Lancaster pays off on return from the Gulf and given the ongoing powerplant replacements on the Type 45 destroyers and the awful delays in building the new Type 26 frigates, the Royal Navy now faces a chronic "escorts gap" over the next several years.
'This perfect storm of a shortage of warships is hardly what you want if you're trying to deter the likes of Vladimir Putin.'
Speaking earlier this year amid a flurry of Russian activity off the British coast, Armed Forces Minster Luke Pollard said: 'Russian warships are increasingly sailing through the English Channel, and every time they do, a Royal Navy vessel will be keeping an eye on them.
'I have every confidence, as should the British public, that our Royal Navy will continue to defend our waters and keep our undersea cables safe.'
The Mail has approached the Ministry of Defence for comment.