Owners of the 175-turbine London Array wind farm off Kent, the biggest offshore farm in the world, and the 108-turbine West of Duddon Sands wind farm off Cumbria have applied to the Marine Management Organisation for permission to carry out urgent repairs.

The London Array was completed in 2013 and West of Duddon Sands a year later. Both use a type of turbine made by Siemens Gamesa, which has admitted that the leading edge of the blades — the part that slices through the air when the turbine turns — is being eroded much faster than expected on some of the machines. A spokesman said that various factors including the “wind speed, the rotor configuration, the amount of rain, and even the size of raindrops” were thought to be behind the problem.

The UK has more offshore wind farms than any other country after supporting their construction with generous renewable energy subsidies to help to meet climate change targets. More than 1,600 turbines operate in UK waters, but critics have long questioned how reliable they will prove in the harsh conditions offshore.

If the issue with the Siemens Gamesa turbines proves symptomatic of a wider problem, it could undermine the economics of building wind farms.

The type of turbine known to be affected has blades spanning a 120-metre diameter. When installed they were among the biggest used offshore. The tips of the giant blades move more quickly through the air than in earlier, smaller models, which may have contributed to the erosion.

Siemens Gamesa said that it had installed more than 950 of the affected model worldwide, but was unable to confirm the number in the UK. Orsted, the Danish company that co-owns both London Array and West of Duddon Sands, said that it was also assessing four other wind farms that used a similar specification of turbine.

Far bigger turbines are now being installed but Siemens Gamesa said that they should not be affected because the potential for the leading edges to erode was identified in 2014 and all machines installed since had extra protection.

Siemens Gamesa said that it would carry out “performance upgrades”.

The affected blades were made in Denmark with glass fibre and balsa. The repair involves glueing a 3mm rubber-like shell onto the damaged areas. The blades are likely to be removed to carry out the repairs.

Work on both wind farms is due to start next month and could take from one to three years. The companies refused to disclose the costs of the repairs, expected to stretch to tens of millions of pounds. They said that consumers would not face any costs.

John Constable, a long-term critic of wind farm costs who writes for the climate-sceptic Global Warming Policy Forum, said: “If this is a part of a general phenomenon, then the implications for the economic viability of wind power in general are very serious.”

A spokesman for Orsted said the problem would not result in any “noticeable impact on the continuing production of clean, green electricity”.