Councils are being "hustled" into allowing development on greenfield land by central Government, the director of the National Trust has said.
Pressure to allow a growing number of housing developments in the green belt are forcing "ill-prepared" councils to hastily re-designate countryside areas instead of building on brownfield sites, Dame Helen Ghosh claimed.
Speaking on BBC Radio 4's The World This Weekend, Dame Helen said that the National Trust is "very concerned" about the issue and is "making representations" to the Government about the impact of its planning reforms on the green belt.
She said: "We are very concerned that the haste with which local authorities - some of them ill-prepared to do so - the haste with which they've been hustled into producing their local plans and the pressure they're under to produce the numbers of houses has forced them, in some cases, to designate greenfield sites, and we are very worried about that and we are monitoring it and making appropriate representations to Government about it.
"There were some positive signs recently in recently-issued guidance from the Department for Communities and Local Government that the incentives to develop on brownfield sites would be strengthened, and some of the arguments that developers make - that it's just too difficult to develop on brownfield sites and you have to do so much investment before you begin - were ones that local authorities should not listen to.
"So we were encouraged by that, but we are still very worried at the number of potential planning permissions that are out there on greenfield sites."
Much of the hostility towards the Government's policies has focused on planning minister Nick Boles.
Responding to Dame Helen's claims, Mr Boles said: "This Government values and protects the countryside.
"The level of Green Belt development has fallen to its lowest rate since modern records began, with a decrease of just 0.02% between 2011 and 2012.
"Councils have had a decade to shape where development should and should not go.
"So far three quarters of local authorities have published a draft plan and local residents should hold slow-coach councils to account."
Dame Helen's claims come after Sir Simon Jenkins, chairman of the National Trust, warned last month that the green belt was "no longer sacrosanct" and called for a planning regime capable of protecting greenfield land around cities.
Sir Simon described protections for the green belt which were hailed by David Cameron when planning reforms were laid out as "virtually worthless".
"We shouldn't have to fight for the green belt in 2014. At the present moment 150,000 applications are in for the green belt," he said.
"This should be absolutely inconceivable."