A report from the rural conservation charity examined the number of homes completed on greenfield land that has been removed from green belt designation over the past 10 years and built in the three years from 2015/16.
Of the 804 units completed on such sites, just 106, or 13 per cent, can be classified as affordable, the study found.
Meanwhile, a total of 706 affordable homes - a slightly higher proportion of 16 per cent - was found for the 4,389 homes built on greenfield land within the existing green belt between 2015/16 and 2018/19.
Combining the number of homes in both categories, the total proportion of homes built on greenfield land on either deallocated or existing green belt comes to 15 per cent.
The research was undertaken by CPRE examining section 106 agreements on applications approved on sites either in the green belt or deallocated from the green belt, which were provided by research firm Glenigan.
The figures relate to greenfield land only, not previously-developed brownfield land in the green belt.
The report also claims that the number of new homes proposed for land to be removed from the green belt, based on figures in adopted or emerging local plans at an advanced stage, is currently 266,000 – which CPRE says is a rise from 220,000 last year.
Only a third of these are likely to be classified as ‘affordable’ according to local policies, says the report.
Of these 266,000 homes, the CPRE said that 120,624 homes are proposed on land to be removed from the green belt in adopted local plans, while a further 145,465 are included in emerging plans.
Pullinger said CPRE was no longer counting local plans in the earliest stages of adoption - last year it claimed that a total of 459,000 homes were on land deallocated as green belt in adopted or emerging local plans.
This year's report also claimed that the number of residential applications on greenfield sites in the green belt is the highest it has ever been, with 155 applications submitted in 2018/19.
It went on to say that the average density of newly-created residential addresses is half as high within green belt areas than outside them - just 14 dwellings per hectare, compared to an average of 31 in non-green belt areas.
CPRE called on the government to reintroduce a clear brownfield-first policy and target funding to ensure urban land can be used for redevelopment.
It also called for measures to promote and enhance the green belt, including a strengthening of the tests required to prove "very special circumstances" to justify building on the green belt.
Tom Fyans, deputy chief executive of CPRE, said: ‘Building homes on the green belt is not the answer to the housing crisis. Indeed, in terms of the green belt, it’s clear that we are reaching a tipping point.
"The increasing number of new homes proposed on the green belt has continued to rise since the report was first undertaken in 2012, despite the fact that these homes are not delivering promised affordable housing.
"We must not allow our green belt to be gobbled up, but instead focus on building affordable homes in which young struggling families can actually live."
The report uses the definition of "affordable" as stated in national planning policy, which includes social, affordable and intermediate housing to rent or buy.
An MHCLG spokesman said: "This report is misleading as the CPRE’s figures refer to plans that may never be acted upon.
"Ministers have said repeatedly that building the homes our country needs does not mean damaging our countryside. It’s a fact the Green Belt is now around 30,000 hectares larger than in 1997.
"Last year only 0.02 per cent of the Green Belt was developed for residential use and often this development is around road and rail infrastructure in place long before Green Belt designation."
CPRE has long claimed that just over one million homes can be built on brownfield land.
*NOTE: this artice was update at 5pm on Monday 14 October to add a comment from the MHCLG