A team of conservationists is carrying out a survey of basking sharks around the South West coast.
Basking shark populations have been decimated by over-hunting
Concern is growing at the shark's dwindling numbers, and researchers want to know more about its habits to learn how to protect it.
The study will be based on a 39-foot yacht, which will sail around the coast of Devon and Cornwall, as well as parts of north and south Wales, Ireland and the Western Isles of Scotland.
It is being carried out by a small crew of conservationists from The Wildlife Trusts, together with volunteers, and is backed by the Heritage Lottery Fund and the National Express Group.
Project leader Colin Speedie urged people to be on the look-out for basking sharks in coastal waters.
But he warned that they can leap out of the water unexpectedly.
"We cannot afford to ignore the presence of this majestic creature in our waters if we are to ensure its survival for the future," he said.
Basking sharks measure up to 12 metres and can weigh seven tonnes
"The survey will undertake crucial work to help the basking shark, while providing many volunteers with the opportunity to observe marine wildlife at close quarters."
Little is known about the basking shark's activities, which makes effective conservation very difficult.
Without accurate population numbers and an awareness of their behaviour, it is difficult to help this internationally endangered species and to ensure that it is safe from harm during its time in UK waters.
Basking shark populations in the UK have been decimated in the past century by over-hunting and they have still not recovered.
It is thought that numbers plummeted because of the high value of the species' gigantic dorsal fin, a delicacy in the Far East.
While hunting is now banned, the basking shark is still at risk from hunting and a variety of other threats, including collisions with boats, entanglement in fishing nets and changes in sea temperatures due to climate change.
Basking sharks measure anything up to 12 metres and weigh up to seven tonnes - larger and heavier than a double-decker bus.
They feed on plankton, filtering 1,000 to 2,000 cubic metres of sea water - equivalent to two swimming pools - per hour.