Vineyard Wind set a deadline of six weeks to receive a key environmental all-clear from US federal authorities, warning US interior secretary David Bernhardt it will be “very challenging” to advance the project in its current form otherwise.

The project was told earlier this month by the US Bureau of Ocean Energy Management (BOEM) that it would delay issuing a final environmental impact statement (EIS) to what will be America’s first large-scale offshore wind farm.

The 800MW Vineyard Wind project is on a tight timetable as it races to enter full service off Massachusetts by the end of 2021, maximising the benefits it can extract from the fading investment tax credit (ITC) federal incentive for co-owners Avangrid and Copenhagen Infrastructure Partners.

The developer repeated its initial comments that a project of its significance is bound to attract thorough scrutiny, but added: "Vineyard Wind has communicated to BOEM that for a variety of reasons, it would be very challenging to move forward the Vineyard Wind 1 project in its current configuration if the final EIS is not issued within, approximately, the next four to six weeks.

"BOEM has indicated they understand the reasons for this constraint and will communicate this to the secretary of the interior, who is responsible for final action on this project. Vineyard Wind has also communicated to the secretary directly about its concerns regarding the delay."

The BOEM delay was one of two simultaneous knockbacks for Vineyard Wind. The second came from conservation commissioners in Edgartown, one of the local authorities that needs to sanction the Vineyard Wind export cable.

They voted against doing so, citing fears among local fishermen, in a decision Vineyard Wind condemned as flawed and has asked state officials in Massachusetts to overturn.

The project already has a number of permits in the bag, and has lined up an array of suppliers from the global offshore wind sector including MHI Vestas, which is due to deliver its 9.5MW turbines.