President Vladimir Putin is set to recommission the virtually undetectable Piranha vessels as part of a £230million military spending spree.
Piranhas, which can lay mines and fire torpedoes, were developed but axed during the Cold War.
A source said: “Putin is breathing life into many old programmes and thinks subs are an effective way of getting what he wants militarily.
“The Piranhas can come extremely close to land and are almost impossible to detect as they are so small.
“They could represent a significant threat to Britain if despatched here.”
A senior defence source said: “There have been several occasions in the last five years where foreign submarines have entered British water.
“They have been detected more by chance than anything else because we have no means of monitoring their activity.”
The most recent saw a fishing boat almost sunk off the Isle of Man when a submarine reportedly got caught in its net in April.
Full article: Britain at ‘SIGNIFICANT risk from Putin’s new undetectable torpedo-firing submarines’ (Express)