Downing Street is said to have overruled Michael Meacher, the Environment Minister, and Mr Cameron's appointment will be greeted with dismay by ramblers' campaigners, who will accuse the Government of giving in to the landowners.
Mr Cameron, a friend of Tony Blair, said before the election that his association's 50,000 members should not panic about a Labour victory over the threat of higher taxes. "New Labour and the CLA have a lot in common and Tony Blair's enthusiasm for the countryside is refreshing," he said.
Senior Whitehall sources said last night that Mr Cameron had "signed up" to the Government's plans, to be announced next week by Mr Meacher, for a statutory right of access for ramblers to go on private land, except where there overriding reasons to protect crops or wildlife.
The choice of a new chairman for the agency - which is being created by merging the Countryside Commission and the Rural Development Commission - has proved a headache for ministers. Jonathan Dimbleby was among those considered, and the short list included Elinor Goodman, the political editor of Channel Four, and a keen horsewoman.
Amid some energetic backstabbing over the post, she was accused of being too close to Number Ten. She is separated from the economic adviser in Mr Blair's policy unit.