IT was billed as one of Bear Grylls’s most audacious challenges yet. The Eton-educated television adventurer had to escape an active volcano in the Pacific by leaping across molten lava and avoiding clouds of “killer” gas.

However, the episode of Born Survivor set on the Mount Kilauea volcano in Hawaii has emerged as faked in a scandal that has embroiled the television industry - and now threatens Grylls’s future TV career.

The white clouds of poisonous “sulphur dioxide” that billowed around the former SAS explorer were, in fact, harmless vapour created by smoke machines. And according to insiders, the red glow of the molten magma which he warned could incinerate him “in seconds” was supplemented by burning hot coals brought in by members of the production team.

This weekend Discovery Channel, which produced the programme, said the trickery had been identified as part of a review of the show.

Last month The Sunday Times disclosed how other parts of the programme, which were sold to Channel 4, were also faked: Grylls stayed in hotels when he claimed to be “a real life Robinson Crusoe” on a desert island; a raft he was shown building to use to escape was, in fact, put together by a team of experts; in another episode the producers shipped horses from a trekking station to pose as wild mustangs.

Now Grylls is in danger of being dropped from the C4 schedules. “If what has been alleged is proven to be true, I think the channel would have to think very seriously about its future relationship with him,” said a senior C4 executive.

Grylls, who once served with 21 SAS Territorial Army Squadron, first came to the public eye as the youngest Briton to climb Everest, at the age of 23, before moving into adventure documentaries.

In the volcano episode, which has not been broadcast on C4, he is filmed amid clouds of white gas seeping from the crust of the lava field. “Look at this, you can actually see the sulphur dioxide seeping out of these vents,” he says. “In high concentrations this gas is a killer.”

But this was “special effects” according to a safety adviser.

“Sulphur dioxide fumes are colourless and you can’t see it, so smoke generators were used off-screen to make the existing fumes seem visible,” he said.

A Discovery insider said the fakery was “unacceptable” and had been identified in an internal investigation. Now the channel is reediting the series. Viewers will hear a disclaimer before each show stating that Grylls receives help from survival experts and health and safety officers.

C4 will decide whether to continue broadcasting Grylls’s programmes after its own investigation into Born Survivor concludes this autumn. It has already decided not to repeat the show, which drew 1.4m viewers in 2006.

A spokesman for Grylls said he felt unable to comment as the investigation was continuing.

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010