Mr Lees, who had been filming on the island since December 14 and was due to return at the weekend, has not been seen since he walked into the forest around 4.30 pm last Saturday to take some final video footage and sound recordings.
His driver waited until 9pm before raising the alarm. The pair had driven out from Mr, Lees' hotel 15 kilometres away at Fort Dauphin .
Search parties have been out every day since Mr. Lees disappeared but have found no trace of him. The forest where he vanished is thick and inaccessible but he had visited the area twice before.
Charles Secrett, executive director of Friends of the Earth, said the likeliest explanation was that Mr. Lees, aged 46, had stumbled and concussed himself or broken an ankle and had been caught out by the rapid tropical nightfall.
"The forest there is so dense that shouting doesn't do any good. You could be 50 yards away from people and they wouldn't hear you. It's also very easy to get disorientated because of the lack of reference points like a landscape horizon."
Mr. Secrett said Mr. Lees had water on him but no food or survival equipment. The island has no dangerous predators and the terrain is flat.
He has been campaigns director of FoE since 1990 and has worked for it both as an unpaid volunteer and a member of staff since the late 1970s. He has been behind several of FoE's most successful campaigns, notably over water purity and beach cleanliness.
Chris Orengo, his girlfriend, was due to arrive on the island today with Jonathan Kaplan, a researcher-photographer who knows Madagascar well. The British embassy has also sent a senior official to Fort Dauphin to co-ordinate searches by police and localvillagers.
Mr Secrett said Mr. Lees would be able to survive for several days with just water. "He hasn't got tremendous experience in rainforest areas but he is a resourceful bloke so we are quietly hopeful that things are going to turn out well."
The Foreign Office said everything possible was being done.