The walk is expected to take two weeks

Lloyd Scott, 41, is attempting to complete the 26-mile underwater trek in an antique diving suit.

Mr Scott set a world record earlier this year by completing the London Marathon in the event's slowest time while wearing the ancient equipment.

The father-of-three from Rainham, Essex, hopes to raise more than �1m for cancer charity Children with Leukaemia by completing the stunt.

He said that his only concern was what to do if he met the loch's mythical monster.

I've been told the big copper helmet could give quite a big head-butt to the monster Lloyd Scott

"If I do meet Nessie, I'm not sure who will be more scared, the monster or myself," he revealed.

"I've been told the big copper helmet could give quite a big head-butt to the monster, so it will be quite nice to make his acquaintance."

The former firefighter will make his way along the underwater shelves and ledges that run around the banks of the loch, about 30ft below the surface, at two miles a day.

He will also have an eight-man team monitoring his progress in a specially designed boat at the loch's surface.

The crew will track the adventurer using the latest GPS equipment and feed him air from an onboard pump.

While 1,300 other runners headed off on Sunday's Loch Ness marathon, Mr Scott took the plunge at Fort Augustus, where the Caledonian Canal meets Loch Ness.

He plans to emerge from the water for the last time in two weeks to cross the marathon finishing line in Inverness.