George Osborne will signal today that the Government is to press ahead with highly controversial reforms to Britain's planning system which environmental groups fear will result in the destruction of rural England.

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Mr Osborne will use his Budget speech to announce that, within days, ministers will publish their final National Planning Policy Framework (NPPF) – which removes the protection given to ordinary countryside, and which presumes that the default answer to development proposals will be Yes.

These represent the biggest changes to the planning system since it was set up by the Town and Country Planning Act of 1947, and effectively change it from an instrument to protect the countryside into an instrument to foster economic growth.

The Chancellor will scupper the hopes of conservationists that the reforms might be watered down, by insisting that growth must trump objections to greenfield development. Senior sources who have seen the finalised document, which was approved by a cabinet committee last Wednesday, said it offered few concessions to environmental campaigners.

There will still be a presumption in favour of "sustainable development" – ensuring the planning system makes it easier to build new homes, offices and factories. Groups like the National Trust and the Campaign to Protect Rural England (CPRE) fear this will give developers the opportunity for a building "free-for-all" in the countryside.

The Government will also refuse to give ground on a demand to keep protection for the "ordinary" countryside which represents the 55 per cent of rural England not covered by such designations as green belts or national parks. In previous guidance, in place since the 1970s, ordinary countryside was given an explicitly recognised value in making planning decisions, but in the new NPPF, this has simply been dropped. "If this goes ahead, it will be the biggest and most harmful change to the planning system since it was established 65 years ago, and a huge threat to the countryside," said Kate Houghton, of the CPRE.

Yet despite rumblings among its supporters, not least many of the four million members of the National Trust, which has campaigned vociferously against the changes, the Government is unrepentant. "If we are going to build a quarter of a million homes a year then there is no way we can do that without building on greenfield sites," said one source. "People don't want to see high-rise flats being built, building in back gardens and the destruction of parkland so, unfortunately, there is very little choice other than building on greenfield sites."

In one concession to those opposed to the new regime, the final NPPF will restore the presumption that priority should be given to building on brownfield rather than greenfield sites. The current draft framework only requires that local development plans "should allocate land with the least environmental or amenity value where practical". But ministers say that previous policy was written in that way to protect old brownfield sites which had become ad hoc nature reserves valued by local communities. "Sadly as a result of the campaign that protection won't be there any more," said the source. "Some people may end up rueing what they have done."

It is understood that Mr Osborne had wanted to publish the final framework at the same time as his Budget in the hope it would attract less attention and criticism than if it was published separately. But the Communities Secretary, Eric Pickles, – who is officially in charge of planning – argued that it would make the Coalition appear to be hiding bad news and make it more difficult in the long-term to sell the new proposals to local communities. It is understood that Mr Pickles's view has prevailed.

Privately, ministers believe the campaign against them has been misguided and counter-productive. They claim that current planning law includes a presumption in favour of development and the only difference with the new guidance is that it has added the word "sustainable". They say the new rules will give far more power to communities to shape development in their areas – as well as the chance of getting new amenities in return for planning consent.