Russian President Vladimir Putin is believed to want to increase the size of his midget submarine fleet

Russian president Vladimir Putin is planning to recommission Cold War-era fleet of midget submarines, which are virtually undetectable.

The Piranha-class vessels can operate very close to the shore and can be used to drop mines or fire torpedoes.

With a crew of nine, the Piranha can approach silently and can even deploy combat divers or up to six special forces.

Putin is believed to be willing to sanction £230billion in military spending.

It is understood that one of the old Piranha vessels has been upgraded to act as a sales platform, to show off the potential of the weapons system to other buyers.

One military source told The Sun: '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.'

Putin's increasingly aggressive stance follows his decision to send Cold War-era bombers on patrol around the North Atlantic, forcing the RAF to intercept.

Also, there have been several reports of unidentified submarines patrolling in Swedish waters. During one incident in October 2014, a semi-submerged black object was photographed while the Swedish navy were searching for a suspected Russian submarine.

The Swedish military said they had 'three credible sightings' of what they described foreign undersea activity in their waters.

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Unlike the massive nuclear-powered vessels, pictured, the midget submarines are virtually undetectable

Over the past 18 months, Russian military forces have been patrolling far more aggressively

President Putin, centre, is keen to project Russian military might across the globe

Incursions by Russian subs was commonplace during the Cold War but the practice was less common.

Now there are suspicions Putin has ordered his underwater fleet to patrol areas

Rear Admiral Anders Grenstad said the armed forces had observed the activity in the Stockholm archipelago and nearby coastal area, but declined to give details of an operation reminiscent of the Cold War, when Sweden's armed forces routinely hunted for Soviet submarines in its waters.

During the search operation the armed forces published a photograph showing a partially submerged object in the water from a distance.

Last October, the Swedish military published this photograph of a suspected Russian mini sub

Grenstad told reporters that he wouldn't speculate on the photograph or sightings except to say the region is 'of interest to a foreign power'.

Grenstad said it had no information about any emergency messages suggesting a Russian mini-submarine had run into trouble in Swedish waters and could be damaged.

The Defense Ministry in Moscow said its submarines and ships have been 'fulfilling their tasks in the world's oceans.' according to plan, Russian news agencies reported.

In 1981, a Soviet sub carrying nuclear weapons was stranded off Sweden's southeastern coast, causing an 11-day diplomatic standoff before Swedish authorities allowed the submarine to return home.

The alleged intrusion prompted a massive naval search by the Swedish navy looking to find the sub

The Swedish navy launched several fast-attack craft to try and drive the subs from their territorial waters