An area three times the size of Birmingham in the West Midlands countryside has been lost to developments in less than two decades, campaigners said.
About half of the region's undisturbed countryside has been blighted by noise and visual intrusion since the early 1990s, according to the Campaign to Protect Rural England.
And the group warned that the Government's proposed new planning regime would lead to further accelerated erosion which could see the entire region losing its green areas in a century.
The CPRE today published maps and analysis revealing a constant intrusion into the English countryside since the early 1960s.
The report found since the early 1990s, the West Midlands has lost 315 square miles (815 square kilometres) of undisturbed land - equivalent to three times the size of Birmingham.
The campaign coincides with the launch today of an inquiry by a cross-party parliamentary group into the future of Britain's housing supply.
It will look at the Government's plans for the biggest house building programme since the 1950s, which involves finding sites for more than 400,000 new homes in the West Midlands.
Critics claim that while three-quarters of these dwellings can be earmarked for brownfield land, at least 100,000 homes will have to be built in the countryside.
Shaun Spiers, CPRE chief executive, said he hoped his organisation's findings would act as a wake-up call to the Government.
"Countryside which is undisturbed by noise and development is vital for our quality of life and well-being," he said.
"It is often said that development will only take up a small percentage of England's total land surface.
"But development of all kinds fragments the countryside, undermining the qualities of tranquillity, openness and immersion in the rural landscape which people most value about it.
"An area will no longer be experienced as truly rural long before half its surface is developed."
The CPRE analysis found Herefordshire and Shropshire were the second and fifth least disturbed counties.
Warwickshire, Worcestershire and Staffordshire however, were deeply fragmented, it said.
At the current rates of loss, much of the region's remaining countryside could be blighted in 120 years by major infrastructure such as new power stations, runways and roads.
The organisation said people's right to have a say over planning decisions on major infrastructure would be stripped away if Government proposals in the Planning White Paper are implemented.
It proposes a new planning regime for deciding nationally significant infrastructure projects, with decisions made by an Infrastructure Planning Commission, rather than elected ministers.
The commission would be expected to approve any application as long as it was deemed in line with the National Policy Statement and it would be left to its discretion to decide whether members of the public would have a right to appear at any public inquiry.
Mr Spiers called on the Government to strengthen its policy to protect the remaining areas of undisturbed land for future generations.
"The countryside is one of our greatest national assets," he said.
"I am sure that the Government wants to protect it but these maps show the current pace of development is seriously eroding our countryside.
"The Government must act across the board to demonstrate that it takes the future of the countryside seriously.
"Unless it does so, for children alive today, much of our remaining undisturbed countryside will become a distant memory in their lifetimes."
A spokeswoman for the Department for Communities and Local Government said it was committed to protecting the green belt.
She said: "Strict planning controls exist on development in the green belt and we have no plans to weaken that level of protection.
"Nationally there continues to be an overall increase in the amount of green belt land. Since 1997, the amount of green belt in England has grown by about 64,000 acres.
"We believe that it makes sense to protect the land that matters most - land of high landscape value, or high biodiversity as well as valuable green space in our towns and cities.
"We are not proposing any changes to existing protection for these areas."