Two-thirds of applications to build renewable energy projects in Great Britain have failed to get through the planning stage over the past five years, hampering efforts to shift towards clean electricity generation.

A study of Britain’s “renewables pipeline” found that 63% of mooted projects were either abandoned, refused planning permission, or an application was withdrawn or ultimately expired between 2018 and 2023. The remainder of the applications were either approved or revised, according to the research by the consultancy Cornwall Insight.

Renewable energy developers have bemoaned the difficulty in gaining planning permission for projects – from offshore windfarms to onshore solar and battery storage developments – and waiting times to connect to Britain’s electricity grid of more than a decade.

The research found that no region of Great Britain had a success rate higher than 20% of projects gaining planning permission. Cornwall Insight said “speculative and duplicate applications” were in part to blame, citing reports that “phantom” energy projects were holding back efforts to decarbonise the grid.

Last year, a report commissioned by Centrica, the owner of British Gas, claimed that some developers were applying to connect projects to the grid that did not even have land rights or planning consent applications.

National Grid and Ofgem, the industry regulator, have attempted to remove “zombie projects” – those that have been approved but have stopped being developed – from the connections queue. The problem has also been blamed for years-long delays to new housing schemes.

Lucy Dolton, the assets and infrastructure manager at Cornwall Insight, said: “It’s clear that an increasing number of the applications submitted are speculative, raising the numbers in the connections queue, and creating obstacles for projects that are mostly ready to connect.”

The amount of electricity generated by the UK’s gas and coal power plants fell by 20% last year to its lowest level since 1957. Renewable energy provided the single largest source of power at 42%, although gas power plants still accounted for 31%.

The Conservatives have set a goal to decarbonise the electricity grid by 2035, while Labour is targeting 2030. Both parties have pledged to slim down the planning regime if they win next month’s general election.

The Conservatives have promised to speed up the average time it takes to sign off a big infrastructure project from four years to one year. Labour has said it will update planning policies and fund extra planning officers.

Dolton added: “The total capacity of projects in the grid connection queue is currently well in excess of what is necessary for net zero generation capacity. However, considering the lengthy process for projects to progress through planning and gain grid connections, and the current volume of projects that are unsuccessful, the amount of this capacity that will ultimately connect could be much lower than the pipeline of projects suggests.”