A boat which overturned in seas off Vietnam during a storm, drowning two British tourists, should have returned to harbour an hour earlier when it got dark.
The pair, who have not been named, died alongside a French man and two Vietnamese people when the vessel capsized in heavy winds with 25 passengers and seven crew members on board.
Other passengers were treated in hospital for shock and hypothermia after the incident in Halong Bay, according to reports.
Mai Nguyen, of tour operator Tropical Sails, said she understood the accident occurred after dark when cruise boats would normally be back in harbour or have anchored in a sheltered spot.
She said: "Normally the Halong Bay management want all boats to be back in the harbour before it gets dark at around 6pm.
"The boat was still out in the bay at 7pm, which is very late to be cruising. I think they were held up because some tourists were still swimming."
The boat was hit by heavy winds as it returned to port on Thursday night.
A Foreign Office spokeswoman said: "We can confirm the deaths of two British nationals in Halong Bay off the coast of Vietnam on September 24 and we are providing assistance to their families."
British Embassy officials travelled to the bay, located in the Gulf of Tonkin some 100 miles east of the capital Hanoi, to help officials contact the two Britons' families.
Halong Bay has become increasingly popular with UK travellers, particularly divers for its coral reefs and freshwater swamp forests.
Sunny Bui, manager of Cruise Halong, one of the tour operators that run boat trips to Halong Bay, said an accident of this scale was almost unheard-of in the area.
"It never happened like this before. There was no warning. (Tour operators) usually hear of typhoons and strong winds in the weather forecast, but this was a whirlwind that only lasted for 15 or 20 minutes, and nobody knew about it."