(Reuters) Plans by Airbus to upgrade its best-selling wide-body jet, the A330, have slipped six weeks behind schedule due to “marginal” development delays at engine maker Rolls-Royce, a source familiar with the matter said on Friday.

Airbus declined to comment on the situation.

Rolls-Royce, whose newly developed Trent 7000 will power the fuel-saving A330neo, was not immediately available for comment.

The Wall Street Journal earlier quoted the head of the airplane’s first customer, TAP Portugal, as saying its first A330neo would arrive in March 2018. Airbus most recently said the A330neo would be delivered by the end of 2017.

When it launched the A330neo in 2014, it targeted first delivery in the fourth quarter of 2017.

Reuters