Britain's best farmland will no longer be shielded from development, proposed new planning rules suggest, reversing a pledge made by the Conservatives before the last election.

Instead, the new National Planning Policy Framework (NPPF), which is generating fierce opposition from countryside and heritage groups, suggests that even "the best and most versatile agricultural land" can be built on in certain circumstances.

The change is evident when comparing the Conservative green paper, Open Source Planning, which was issued in 2010 and foreshadowed how a Tory Government might reform the planning system, and the draft NPPF itself, which was released in July and is now open for consultation.

The earlier document says on page 20: "We will introduce into our national planning framework rules preventing the development of the most fertile farmland, in all but exceptional circumstances."

However, in the new draft, ministers have backpedalled over protecting such land. The document says, in paragraph 167: "Local authorities should take into account the economic and other benefits of the best and most versatile agricultural land."

The change is part of a substantial shift in emphasis that the Government is trying to enforce in the planning system, from protecting the countryside and the natural environment to facilitating development.

At the heart of the new guidance, which will replace 1,300 pages of planning rules with fewer than 60, is "a presumption in favour of sustainable development" and the idea that "the default answer to development will be Yes." This has attracted sharp and sustained criticism from groups such as the National Trust and the Campaign to Protect Rural England (CPRE), which has labelled the new framework a potential "developers' charter".

They are alarmed that the pledge to protect the countryside for its intrinsic value, which has been part of the modern planning system since its inception in 1947, has been dropped in the new framework, as has the presumption that brownfield or derelict, former industrial sites, should be built on before greenfield land in the countryside.

The removal of specific protection for the best agricultural land is part of the shift and was criticised by the CPRE yesterday.

"It's a good example of how the NPPF is all about the short-term economics and not about the longer-term issues like food security or what we want the countryside to be for," said a CPRE spokesman.

"We think that strong protection for best quality agricultural land should be an absolutely critical part of sustainable development, given that such land is a finite resource and the pressure on food and farming from wider global pressures of climate change and population growth.