The two turbine giants were put on the defensive by a report from Maurice Rosenthal, ING Groep NV analyst, alerting investors to the problem, which appears to affect a component joining the blades to the hub of older 660kW turbines from both companies. The report gained broader attention from a Bloomberg story and weighed on Vestas and Gamesa shares Wednesday.

Danish wind leader Vestas says it began notifying customers in autumn 2010 and has already scanned its entire fleet of V47 machines, most of which were deployed about a decade ago. It found the problem affected only about 4% of machines, not 30% as suggested by Rosenthal.

Remediation work, which is necessary because of the risk of the blade joint loosening, has been completed, at customer expense for turbines off warranty, for most of the affected machines, the company says.

"Gamesa has known about a failure on one of the components incorporated in the G4X by means of its own customers, which apparently were informed by the manufacturer in charge of its design and supply," a spokeswoman for the Spanish turbine group says, noting that the component was not from Gamesa. Vestas supplied the component in question.

Gamesa says it is offering its affected customers an inspection to determine the extent of the problem and proposes a solution that it says will solve the problem and extend the life span of the turbines up to 30 years.

Sources familiar with the situation say Gamesa's estimate of the percentage of its fleet affected is similar to that of Vestas.

Vestas disclaims any financial responsibilty to Gamesa for having designed the component and says it is up to Gamesa on how it handles the issue with its customers.

Vestas also questions the timing of the analyst's report, given that the problem emerged 18 months ago and has been largely resolved. It follows unfortunate headlines in the last week regarding a fire in a new Vestas turbine in Germany and an accident at the Macarthur Wind Farm in Australia.

Rosenthal suggests a remediation cost of €27,000 ($35,483) per megawatt of capacity, a number Vestas disputes, although the company did not offer its own estimate.

Bloomberg reports that Iberdrola had purchased at least 1,000 G47s and Acciona had at least 500. NextEra Renewable Energy reportedly purchased upwards of 1,000 V47s.

Iberdrola and NextEra did not respond to requests for comment.