Norwegian Air International (NAI) has welcomed a “long overdue” approval from the US Department of Transportation for its foreign carrier permit.

Several large US and European carriers had waged a two-and-a-half year battle to block the award, arguing that NAI’s Irish registration was a flag of convenience.

However, their case was dealt a major blow in April, when the DOT stated that the “NAI appears to meet DOT’s normal standards for award of a permit and that there appears to be no legal basis to deny NAI’s application”.

The airline now awaits a permit for its British unit, Norwegian Air UK, which it appears to expect following an announcement that it is to boost flights between Britain and the US by 50 per cent next summer.

“We would like to thank the many airports, airlines, industry and business groups, politicians and members of the public in Europe, the US, and Ireland in particular who have offered us their continued support over the last three years.

“While the delays Norwegian have faced have been unfortunate and unnecessary, ultimately the decision now made by the US DOT finally paves the way for greater competition, more flights and more jobs on both sides of the Atlantic,” said a spokesperson for the airline.

Monarch Aircraft Engineering will also celebrate the deal, as in 2014 it started providing line maintenance for Norwegian 787s based in London Gatwick.

In addition, British Airways has performed base checks of Norwegian 787s at its Cardiff technical facility.

Both companies’ services fall under a GoldCare aftermarket deal that Norwegian and Boeing updated this summer. Under the deal, Norwegian will keep adding 787-9 aircraft to the original GoldCare contract agreed in 2012.

The airline expects to have around 40 787s by 2020.