Five British nationals have died on board a whale watching boat which sank off the coast of western Canada.

Foreign Secretary Philip Hammond said his thoughts are with the family and friends of all those affected by the "terrible accident" which took place on Sunday afternoon off the coast of British Columbia.

"It is with deep sadness that I can confirm five British nationals have lost their lives when the whale watching boat they were on sank off western Canada on Sunday.

"My thoughts are with the family and friends of all those affected by this terrible accident.

"Consular staff in British Columbia are supporting the family members of those who have died and we will remain in close contact with Canadian authorities as further information becomes available," Mr Hammond said in a statement.

The boat, operated by Jamie's Whaling Station, a local tour company, got into difficulty on Sunday afternoon local time in the tourist community, which is a popular destination for whale watchers.

A major search and rescue operation was launched, with the boat partly submerged eight nautical miles from Tofino, on Vancouver Island, according to reports.

John Forde, who runs The Whale Centre, another whale watching operation, said he did not know what could have caused the incident.

''Over the course of a season and years we take out thousands and thousands of people on these trips in conditions similar (to) today. I have no idea what the issue was or what actually happened,'' he said.

Jamie's Whaling Station was one of the first of its kind off Vancouver Island and had been around for many years, Mr Forde said.

In a statement on the Jamie's Whaling Station website, owner Jamie Bray wrote: ''It has been a tragic day. Our entire team is heartbroken over this incident and our hearts go out to the families, friends and loved ones of everyone involved.

''We are doing everything we can to assist our passengers and staff through this difficult time.

''We are co-operating with investigators to determine exactly what happened.

''In the meantime, we want to extend our most sincere thank you to the first responders, rescue personnel, and everyone from Tofino and the local First Nations communities who assisted with the response efforts.

''We will provide further updates when information becomes available.''

There were 27 people on board the boat.

Acting Sub-Lieutenant Melissa Kai, of the Joint Rescue Coordination Centre in British Columbia, said the rescue operation had finished at 11.10pm local time (8.10am GMT).

She said: "Of the 27 individuals on the vessel when it sank, 21 were rescued. Five were located without vital signs and one remains missing."

A Royal Canadian Air Force Cormorant rescue helicopter had been at the accident site within 30 minutes of the emergency call, she added, and three coastguard ships had joined other local boats in a rescue attempt.

Tofino is an "outdoorsy retreat" which attracts surfers and families, according to Lonely Planet.

On its website, Lonely Planet says: "Transforming from resource outpost to hippie enclave and now eco resort town, Tofino is Vancouver Island's favorite outdoorsy retreat.

"It's not surprising that surf fans, families and city-escaping Vancouverites keep coming: packed with activities and blessed with stunning regional beaches, the funky community sits on Clayoquot Sound, where forested mounds rise from roiling, ever-dramatic waves."

It was a clear and sunny day when the 20-metre Leviathan II made a mayday call.

Video footage online shows the boat bobbing vertically in the water, with what appears to be a large section of it submerged below the waves.

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