One of two Danish crew members missing from a capsized barge has been found dead in the hull after an overnight collision with a British cargo vessel in a busy Baltic Sea route off Sweden.

The 55-metre Karin Hoj barge and 90-metre Scot Carrier were sailing in the same direction off the island of Bornholm when they crashed in fog and darkness at 3.30am, maritime agencies in Denmark and Sweden said.

At least nine boats and a helicopter combed waters near the wreckage-strewn site, while the overturned Karin Hoj was towed away to shallower waters so Swedish divers could investigate the hull without risk of being dragged down.

It is impossible to survive for long without protective gear in the sea temperatures of around 4 Celsius, but there had been hope the pair might be alive inside an air pocket within the hull.

However, the Swedish Maritime Administration (SMA) and police said one person was found dead on the boat, while the other was still missing.

Though there was fog at the time of the accident, the Danish Meteorological Institute said it was not dense and conditions were not unusual for the time of year.

The Swedish Prosecution Authority said in a statement a preliminary investigation was under way into several suspected crimes: gross negligence in maritime traffic, causing death through negligence, and marine intoxication.

It said two suspects, one British and one Croatian, had been take into custody and it could provide no further details due to the preliminary nature of the probe.

With events seeming to have unfolded fast, the Danish Defence's Joint Operations Centre (JOC) said there was no sign lifeboats or other emergency equipment were used by the Karin Hoj's two-person crew.

"I can confirm an accident has happened but I do not know the circumstances," Soren Hoj, managing director of Rederiet Hoj which owns the Karin Hoj, told Reuters.

The barge was sailing from Sodertalje in Sweden to Nykobing Falster in southern Denmark, while the Scot Carrier was sailing to Montrose on Scotland's east coast.

The British-registered Scot Carrier at the scene of the collision

The British ship's owner Scotline said it could not comment on details but was assisting local authorities.

The SMA said the Danish vessel was empty of cargo and it was unclear what if anything the British ship was carrying.

The waters between Sweden and Denmark see large amounts of maritime traffic. Collisions are rare though in 2006 a small Danish vessel sank after a crash with a Swedish boat, with all the crew rescued alive after bailing overboard.