Only one in 10 homes built on green belt land in England in the last decade has been classed as affordable, undermining the case for the development of supposedly protected areas, according to countryside campaigners.

The Campaign to Protect Rural England also found that only a third of the 266,000 future new homes currently proposed for green belt land around the country are likely to be affordable. In a report published on Monday, called Space to Breathe, the charity said most green belt developments were inefficient, with only 14 homes for each hectare instead of the 31 for each hectare that is the average on other land.

Ministers face a dilemma over building on the green belt, as successive governments have tried to release land from the protected designation in order to ease the housing crisis. But many key Conservative seats have areas of green belt land where local people are resistant to development.

The government was criticised last week over plans that would raise the cost of building new social housing, despite the slow rate of current building and the acute housing need.

The CPRE argues that brownfield land offers better opportunity for more housing, and has identified enough such land to build 1m new homes around the country. Tom Fyans, the deputy chief executive of the charity, said: “It’s clear we are reaching a tipping point [on the green belt as] the increasing number of new homes has continued to rise since 2012, despite the fact that these homes are not delivering promised affordable housing.”

However, rural businesses are keen to ensure that housing development is still possible and fear current planning rules are too restrictive. Many young people are unable to find suitable and affordable homes in the countryside, putting strain on communities.

Tim Breitmeyer, the president of the Country Land and Business Association, which represents 30,000 rural businesses, said: “We want more opportunities for young people to stay in or relocate to rural communities, but they are put off by the lack of affordable homes. There is plenty of opportunity for sympathetic and proportionate housing development across the country, held back by the current planning system.”

He called for reforms to the planning rules and said housing developments need not destroy the character of the countryside. “Villages must be allowed to grow organically, with small quantities of well-designed new homes aimed at local people,” he said. “As well as strengthening the rural economy, it will ensure the countryside is positively contributing to solving the national housing crisis.”