Five Britons have died and a sixth person, believed to be Australian, is missing and feared dead after a whale-watching boat carrying 27 people capsized near Vancouver Island off Canada's Pacific coast.
The search and rescue operation was called off on Monday (local time) after 21 survivors were plucked from the water, Lieutenant Commander Desmond James of the Coast Guard's rescue centre in the provincial capital Victoria told AFP.
A Royal Canadian Mounted Police dive team later took over, scouring the ocean bed and the rugged coastline nearby for any sign of the missing sixth person, who has yet to be identified.
AAP has been told by the family of a Sydney man, 27, that he was on the boat with his girlfriend and her family when it went down.
Her father was one of five British citizens confirmed dead.
Officials admitted there was little chance the passenger would be found alive, almost 24 hours after the Leviathan II sent out a distress call to say it was sinking.
"We still remain hopeful, but we have to assume the worst," said RCMP Corporal Janelle Shoihet.
Federal Transportation Safety Board (TSB) investigators, meanwhile, secured the site of the wreckage.
In London, Foreign Secretary Philip Hammond said the four men and a woman who perished in the capsize were all British citizens.
"My thoughts are with the family and friends of all those affected by this terrible accident," Hammond said in a statement.
Two of the victims were Canadian residents while the other three were tourists, said the British Columbia coroner's office.
They were aged 18 to 76.
The Leviathan II went down some 12 kilometres off Tofino, a resort town on the western edge of Vancouver Island.
Owned by Jamie's Whaling Station and Adventure Centres, the 20-metre cruise vessel was reportedly out on one of its last tours of the season, which ends October 31.
"This particular boat has done this exact same trip for 20 years twice a day. Yesterday was no different than any other day," said tour company owner Jamie Bray.
But for reasons yet unknown, the ship capsized in waters less than seven metres deep, its bow remaining visible above water.
Locals told Canadian media the vessel may have hit rocks.
"It's much too early to say what the causes of this accident might be," said TSB Director of Marine Investigations Marc Andre Poisson.
A company spokeswoman said the incident happened "so quickly" that the crew was unable to send out a distress signal.
Once in the water, the crew set off flares that attracted the attention of local aboriginal fishermen nearby.
Eighteen people were hospitalised, several suffering from hypothermia, media said.
"The response here has been nothing short of phenomenal, the way that people are bringing out blankets and clothing and food, donating what they can and offering all of the services that they have," Tofino Mayor Josie Osborne told broadcaster CTV.
"Tofino's thoughts and prayers are with passengers, crew, emergency responders and their families. Thank you all for your messages of support," she wrote on Twitter.
A "shocked and saddened" prime minister-elect Justin Trudeau offered his thoughts and prayers to passengers, the crew and families of the victims.
An employee who answered the phone at Jamie's Whaling Station said the company was focused on the passengers and crew.
Tofino is a popular surfing and whale-watching town near the Clayoquot Sound UNESCO Biosphere Reserve.
Jamie's Whaling Station, one of the area's largest tourist boat outfitters, was hit by a deadly tragedy once before, in 1998.
According to TSB records, two of the four people aboard a whale-watching ship, Ocean Thunder, died after a "large swell wave struck the boat from the port side."