Half of councils could be forced to approve construction in the countryside as they will not have plans in place to guide development when new planning rules come in, campaigners have claimed.
Under controversial reforms to the planning system, which focus on a presumption in favour of sustainable development, there is a default "yes" to projects if councils do not have up-to-date "local plans" in place.
The plans set out a local authority's priorities and strategy for delivering housing, economic growth, infrastructure, community facilities and protection of the environment and green spaces.
But research by the Campaign to Protect Rural England (CPRE) suggests that 48% of English councils will not have plans in place by April, when the reforms are expected to take effect.
The CPRE warns that any area without a plan could be "up for grabs" by developers, leading to a "horror story" for England's unprotected green spaces.
And it says the problem is made worse by evidence of cuts to local authority planning budgets, which is leaving councils struggling to get plans in place in time.
Shaun Spiers, chief executive of CPRE, said: "All local authorities need to get their plans in place. But there also needs to be enough time and money available for them to do this and to consult with their local communities."
The Government says changes to the planning system, which slim down more than 1,000 pages of policy to just 52, are needed to boost growth at the same time as giving communities more say in their local area and protecting the environment.
But concerns have been raised that the proposed reforms could lead to a return to damaging development and urban sprawl.
Last week Planning Minister Greg Clark insisted the presumption in favour of sustainable development was not intended to introduce a "loophole" through which development could be imposed on local communities.