The UK’s transition to net zero is under threat as delays in approving new infrastructure put billions of pounds of investment in offshore wind schemes and other vital upgrades at risk, big ports have said.

The British Ports Association (BPA) has written to the government and Labour calling for action to clear the backlog of harbour orders, the legislation needed for ports to make infrastructure changes to support offshore wind projects.

The shadow energy security secretary, Ed Miliband, this week promised Labour will take lead on global efforts to tackle the climate crisis if it wins Thursday’s election.

Port authorities hope to take advantage of the growth of the offshore wind industry – seen as vital to hitting the UK’s legal commitment to net zero carbon emissions by 2050 – by upgrading their infrastructure to allow the assembly and maintenance of turbines on site.

Some ports, however, have been waiting for nearly four years for orders to be signed off, more than four times longer than expected, while other schemes have failed to get off the ground over fears the delays will make them unviable.

The BPA, whose members own and operate 350 ports and facilities, has estimated that it could take nearly nine years to clear all existing orders, while saying that applications are likely to increase in number in the coming years to meet growing demand for offshore wind schemes.

Harbour orders are legislative consents given to ports to construct new infrastructure by the Marine Management Organisation (MMO), a public body.

Ports also require harbour orders to give them new legal powers, enabling them to borrow funds or enter joint ventures, which are essential for the planning of facilities such as upgrades to allow work on turbines or cruise terminals.

The BPA director of policy and external affairs, Mark Simmonds, said significant port development was needed in the coming years to meet the growing demand for offshore wind, as well as growth in cargo and cruises.

“It is extremely regrettable that a single point of failure in processing relatively simple consents at a regulator can threaten billions of pounds of port investment and ultimately put our net zero ambitions at risk,” he said.

The BPA has told Labour that its £1.8bn manifesto pledge to upgrade UK ports could be held back unless immediate action is taken to speed up consents.

The number of approvals has dropped off significantly, with only four harbour orders receiving the green light in the past two years, compared with nine in 2021.

Falmouth Harbour, which has been waiting almost two years to secure a new order, said the delays were preventing it from progressing plans to tap into floating offshore wind projects in the Celtic Sea.

The harbour’s chief executive, Miles Carden, said: “The significant time delays are a massive distraction.”

The Port of Southampton submitted its application for new powers in June 2020 but has still yet to secure an order, and the Port of Plymouth has been waiting three years for MMO approval. Industry experts believe the body’s lack of legal resources is behind the delays.

Associated British Ports, the country’s biggest port operator, said the delays were unacceptable and holding up a number of its important operational improvement projects.

Tim Morris of the ABP said: “Without more capacity at the MMO or alternative routes to process applications, the potential for ABP and other port operators to grow our capability to handle the trade Britain needs and play a greater role in the energy transition risks being lost.”

Offshore wind provided about 17% of the UK’s energy in 2023, with the government targeting 50GW by 2030.

A report by Renewable UK last year estimated that investment of around £4bn would be needed at ports to produce 34GW of energy from floating offshore wind by 2040.

Lara Moore, the head of marine at the law firm Ashfords, said it was aware of projects failing at an early stage because they were unlikely to obtain consents within the two- to three-year window they needed to become operational.

“For offshore wind customers who are trying to bring in turbines, you’ve got to be able to react. You can’t say to a prospective customer, ‘yes, we are really interested but we can’t start for another four years because we are waiting for the consent’,” she said.

A spokesperson for the MMO said the application process typically takes about three years.

“The present volume of applications and the resulting perceived delays were created by the large volume of applications submitted ahead of two price increases to harbour order fees in 2022 and 2023,” the spokesperson said.

“The processing of these applications is complex and time consuming and the MMO is presently not fully cost recovering for work undertaken for this function.”

The Department for Transport said it was working with the body and ports to improve the process.