Paul Hellyer decided to take the New Year's Day flight he was booked on back to Britain, just hours after learning that Mr Lees, the 46-year-old campaigns director of Friends of the Earth, had disappeared in dense tropical forest more than 12 hours earlier.
"I had to make sure the British ambassador was alerted in the capital and tell Andrew's partner and relatives he was missing,'' said Mr Hellyer. "It was impossible to make any telephone calls to anywhere from where we were staying.
"I faced an agonising decision but I felt I made the right one at the time and I still do. I knew when I left that local people were taking his disappearance seriously.'' But Mr Hellyer said he had become increasingly worried and found it hard to sleep at night.
Fears for Mr Lees' life were mounting yesterday as police tracker dogs flown down from the capital Antananarivo joined the search in a strip of coastal forest. He will have been missing for exactly a week today.
Mr Lees and his friend Mr Hellyer had travelled widely on the island in four-wheel-drive vehicles, Mr Lees filming with a video camera and Mr Hellyer taking still photographs. They last saw each other at lunchtime on New Year's Eve, less than 24 hours before they were due to take a flight from Fort Dauphin to Antananarivo.
"Andrew was very enthusiastic,'' said Mr Hellyer. "He felt the holiday had gone well and he had a big smile all over his face.'' They went their separate ways in the afternoon and, unbeknown to Mr Hellyer, Mr Lees paid a final visit to the coastal forestarriving one and a half hours before nightfall.
His driver raised the alarm, after darkness fell, when he had not returned to the vehicle. Mr Hellyer said he was puzzled by Mr Lees' absence from a New Year's Eve party that night and the next morning found his bed had not been slept in. He then learnedthat the police had already been told about his disappearance by the driver and searches had begun.
"It seemed that all that could be done around there was being done, and I had to tell people outside who couldn't be reached by telephone," Mr Hellyer said last night.
Friends of the Earth director Charles Secrett said: "With no further news we remain desperately worried but still believe there is hope this crisis will end happily.''
Mr Lees' girlfriend of eight years, Chris Orengo, flew to Madagascar earlier this week with another of his friends, Dr Jonathan Kaplan, to join the search. The British ambassador, Peter Smith, has asked the Madagascar government to do all it can to help and a reward has been offered.
Mr Lees, who has worked for Friends of the Earth for nearly 20 years, disappeared in the dense, dry forest on his fourth visit to the area during his Madagascar holiday, which began on 14 December.
"It's extremely difficult to move through, more a question of crawling than walking,'' said Mr Hellyer, 38, a travel photographer from north London. "Once you're in it looks the same in every direction - there are very few landmarks.''
The two men had flown to Madagascar for the holiday because of the spectacular variety of plants and animals which are unique to the large island nation.
Mr Lees had been especially enthusiastic at seeing a tiny chameleon in the coastal forest.
But they had extra interest in this particular area ten miles south west of Ford Dauphin, believing the mining giant Rio Tinto Zinc could soon start a mineral sands extraction project which would destroy much of the ecosystem.
Mr Lees was informally sounding out the prospects for campaigning against the scheme. The World Bank had been interested in the project but had already withdrawn in the face of opposition from Friends of the Earth and other environmental groups. Rio Tinto Zinc said it had no plans for mineral extraction for several years, and wanted a dialogue with organisations like Friends of the Earth over safeguarding the forest.
Mr Hellyer believes Mr Lees' disappearance has nothing to do with the mining scheme or any local people. `'Those we met in the remotest places were always smiling and friendly,'' he said.