Sir David began presenting natural history programmes in 1954
The broadcaster Sir David Attenborough has become a patron of a group seeking to cut the growth in human population.
On joining the Optimum Population Trust, Sir David said growth in human numbers was "frightening".
Sir David has been increasingly vocal about the need to reduce the number of people on Earth to protect wildlife.
The Trust, which accuses governments and green groups of observing a taboo on the topic, say they are delighted to have Sir David as a patron.
Sir David, one of the BBC's longest-standing presenters, has been making documentaries on the natural world and conservation for more than half a century.
In a statement issued by the Optimum Population Trust he is quoted as saying: "I've never seen a problem that wouldn't be easier to solve with fewer people, or harder, and ultimately impossible, with more."
The Trust, which was founded in 1991, campaigns for the UK population to decrease voluntarily by not less than 0.25% a year.
It has launched a "Stop at Two" online pledge to encourage couples to limit their family's size.
Other patrons include Jonathan Porritt, chairman of the UK Sustainable Development Commission, and Dame Jane Goodall, founder of the Jane Goodall institute.
BBC environment analyst Roger Harrabin said population was a fraught area of debate, with libertarians and some religious groups vehemently opposing measures by governments to influence individual fertility.
In turn, the Trust accuses policy makers and environmentalists of conspiring in a "silent lie" that human numbers can grow forever with no ill-effects.
In January 2009, Sir David revealed that he had received hate mail from viewers for not crediting God in his nature programmes.
His most recent documentary focused on how Charles Darwin came up with the theory of evolution and why it remained important.