DUBAI/ABU DHABI  - Abu Dhabi International Airport's new terminal is unlikely to meet its new deadline of opening this year, further delaying the multi-billion dollar project, five sources familiar with the matter told Reuters.

The 10.8 billion dirham ($2.94 billion) Midfield Terminal project, which will increase the airport's passenger handling capacity to 45 million a year, has already faced a series of delays from the initial July 2017 opening.

The opening is now scheduled for the fourth quarter this year but three sources said that was unlikely as more time was needed to complete the construction. Two mores sources said it would open in 2020 with one of them saying by next October.

Operator Abu Dhabi Airports did not respond to an emailed request for comment.

The federal aviation regulator said in September it would finish certifying the terminal by the end of the year.

The project is part of a series of multi-billion investments by the government of Abu Dhabi to diversify its oil-based economy which includes developing the tourism industry.

The terminal was devised when Abu Dhabi's Etihad Airways was rapidly expanding to be a major global airline.

The state-owned carrier has since significantly trimmed its ambitions to focus on origin and destination traffic as opposed to connecting passengers through its Abu Dhabi hub.

One of the sources said the scaling back of Etihad's operations meant there was no longer an urgency to open the new terminal.

Etihad now plans to set up a low cost airline with Air Arabia in Abu Dhabi.

In neighbouring Dubai, the government last year put on hold its $36 billion expansion of a second airport, Al Maktoum International.

Officials have said the airport would one day be able to handle over 260 million passengers a year, replacing main airport and Emirates hub Dubai International.

($1 = 3.6727 UAE dirham)

(Reporting by Alexander Cornwell & Stanley Carvalho; Editing by Emelia Sithole-Matarise) ((Alexander.Cornwell@thomsonreuters.com;))