Russian submarine Sweden
A photo showing an object travelling southwards at a speed of one knot inside Swedish waters released by the Swedish military. Reuters

A Swedish navy admiral has claimed that a suspected Russian submarine, which was spotted in waters off Stockholm in October 2014, sparking a maritime hunt reminiscent of Cold War movies, was actually a workboat.

Sweden's armed forces however played down the claim, saying they stood by their previous assessment that the country's territorial waters were violated by a "foreign power".

Rear Admiral Anders Grenstad told a local newspaper that a six-month investigation has concluded that a mysterious object sighted in the Stockholm archipelago in October was a civilian boat.

"The analysis has shown that the photograph taken in Stockholm's inner archipelago was of a smaller boat," Grenstad, told Dagens Nyheter newspaper.

The admiral, who was the deputy chief of the large-scale search operations launched after the sighting, identified the vessel as a 10.5m long, white plastic boat named "Time Bandit".

The daily said the Armed Forces have subsequently classed down the observation from "possible submarine" to "non-submarine" and reported their conclusions to the Swedish government last week.

However the Armed Forces later said the investigation was still ongoing, with a final report on the incident due later in spring.

"We can already observe that the assessment made by the Government and Armed Forces on November 14 2014 remains in full," the military said in a statement.

In November 2014, Stockholm said it had "unambiguous" evidence that a foreign submarine had entered its waters.

"There is no doubt, we have excluded all other explanations," Supreme Commander Sverker Göranson said. "Swedish territory has been seriously and unacceptably violated by a foreign power."

The military said its sensors made a "decisive observation" leading to the conclusion that a vessel had entered Swedish territory but did not provide further details, citing national security reasons.

"The analysis cannot determine the nationality of the intruder," the Armed Forces said .

All fingers were pointed at Moscow as, a few weeks earlier, amid growing tensions between the West and the Kremlin, the mysterious object was photographed by retired naval officer Sven Olof Kviman close to the coast.

Early reports suggested that the unknown vessel could have been a damaged Russian submarine, a U-boat.

Despite launching its biggest military operation in years, Sweden's hunt proved unsuccessful and was called off after a week, with Stockholm saying the vessel had probably escaped into the Baltic Sea.

Kviman told Dagens Nyheter he was sure what he saw and photographed was a submarine.

"It is completely impossible that we have got this wrong, it would mean both my wife and I were colour blind. Time Bandit, at a length of 10 metres, is of a completely different size to the submarine. I saw the submarine above water: the bow, stern and tower. It is always difficult to determine the size, but it was around 20-30 metres long," he told the paper, The Local reported.