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An inventor's £160,000 crowded-funded submarine has sunk "in 30 seconds" after the vessel suffered technical problems and went missing overnight in Copenhagen Harbour.

Owner Peter Madsen, who designed and built the UC3 Nautilus before launching it in 2008, confirmed he was safe and well.

The Danish inventor said the problem was with a ballast tank valve but the matter became "very serious" as he tried to repair it.

"I took about 30 seconds for Nautilus to sink and I could not shut up or something," he said in a rough translation of local media.

(Image: Peter Madsen) (Image: Peter Madsen)

"But that was probably very good, because I'd still been down there."

Local TV network TV2 said that Mr Madsen had told them he was alone on board.

However, Swedish police told the station a Swedish woman disappeared in connection with the submarine, saying she was a journalist on board the submarine.

The journalist's boyfriend was the one who raised the alarm when the boat did not return on time, the station said.

Divers have been sent to the sunken sub.

Two rescue helicopters and three ships from the Danish Armed Forces were looking for the privately built submarine, built at a cost of 1.5million DKK, in Copenhagen Harbour this morning.

But Defence had called on Danes who had access to a boat with sonar equipment to help look for the sub.

(Image: Peter Madsen) (Image: Peter Madsen)

"The more we are looking for him, the better," Defence Commander Christer Haven told TV2 .

The U-boat was reported missing at at 2.30am when it failed to return as planned after leaving the dock at 8.30pm last night.

The owner of the sub is inventor Peter Madsen, who designed and built the vessel before launching it in 2008.

Forsvaret tweeted at 9am today (UK time): "Private submarine Nautilus found sailing in Køge Bay."