A Danish inventor rescued after his home-made submarine sank has been charged with killing a female Swedish journalist who had been on board.

Peter Madsen's sub sank in a bay near Copenhagen on Friday, but he denies the killing and says he returned the 30-year-old to land at 10.30pm on Thursday.

However, police have so far been unable to track her down.

Divers have been sent down to the submarine but have not been able to gain entry.

Mr Madsen escaped on Friday afternoon after the 18-metre UC3 Nautilus, considered to be the largest of its kind in the world, developed problems in Koge Bay, south of the Danish capital.


Image: Helicopters and ships were involved in the search and rescue operation

Two helicopters, three ships and members of the public were involved in the search after the journalist's boyfriend reported the sub had not returned from the trip, which had apparently set sail from Refshale Island at 7pm on Thursday.

Mr Madsen escaped the sinking submarine on a private boat and told Denmark's TV2 channel there had been a problem with a ballast tank.

He said it took about 30 seconds to sink: "I couldn't close any hatches or anything. But I guess that was pretty good because otherwise I still would have been down there."

"Whether the woman was on board the submarine at the time of her disappearance is unclear," Swedish police said in a statement.

Authorities said they might have to tow the submarine to port before they can get inside.

Image: Inventor Peter Madsen (right) denies killing the female journalist

The journalist, who has not been named, had been writing about Mr Madsen and the submarine, according to Danish and Swedish media.

Copenhagen police are appealing for anyone who may have seen her to contact them.

They described her as "160cm tall, 56kg, 30 years old, with reddish-brown hair and green eyes".

She was wearing an orange blouse with long sleeves, a black and white floral skirt, nylon stockings and white tennis shoes.

According to a profile on TedxVilnius, Mr Madsen is "an explorer, who excels in making fantastic dreams come true" and is also said to be working on designing a spaceship.