Vast swathes of the Westcountry will be at greater risk of urban sprawl under proposed changes to the planning system, rural campaigners warn.

Most of Cornwall and almost 60 per cent of Devon are vulnerable to development under the Government's controversial shake-up, according to the Campaign to Protect Rural England (CPRE).

The campaign group has issued detailed maps of land outside of national parks, Areas of Outstanding Natural Beauty and green belt, which it sees as being under threat.

It joins a growing rebellion against the New Planning Policy Framework (NPPF), arguing that enshrining a "presumption in favour of development" in planning policy will give a licence to pave over the countryside.

One senior planner in Cornwall said "comfortably off" campaigners failed to recognise the dire need for housing in the region and misunderstood a key safeguard in the reform – that local plans overrode the shift in emphasis towards approval.

But St Ives MP Andrew George, rural spokesman for the Liberal Democrats, said major changes were needed, including replacing the controversial "presumption" with a "balance of judgement at local level".

The Government says changes to simplify the planning system, reducing more than 1,000 pages of policy to just 52, are needed to boost growth, give communities more say in their local area and protect the environment.

But the CPRE says ministers have provided no reassurance that the final NPPF, due to be published in the coming weeks, will recognise the value of the wider, undesignated rural landscape.

Fiona Howie, head of planning at the CPRE, said: "If we are to avoid damaging the character of rural areas by making it easier for inappropriate, speculative building to take place – a bungalow here, a distribution shed there – decision-makers must be encouraged to take account of the intrinsic value of the wider countryside."

Mark Kaczmarek, Cornwall Council's cabinet member for planning, said its soon-to-be-approved Core Strategy would dictate where 48,000 new homes were built over the next 20 years, not large housebuilders. He predicted that urban areas would expand, through exception sites on the edge of towns and brownfield land, but that "sporadic" developments would be excluded unless communities said otherwise.

"There will definitely be greenfield sites used as there is no option in certain circumstances," he added.