Qantas is pushing ahead with plans for non-stop flights from Sydney to London, asking rival manufacturers Airbus and Boeing to present their “best and final offer” by August for planes capable of the 21-hour trip.

“Hopefully by the end of the year … we will come to a conclusion one way or another,” Qantas chief executive Alan Joyce said on Monday, on the sidelines of an airline industry conference in Seoul.

“If the business case works, we will put in an order.”

Mr Joyce said Qantas was aiming for the planes to be delivered from late 2022, with the first Sydney-London flights likely in 2023.

The route would be the world’s longest commercial flight and Qantas is examining A350 and 777X models.

Trans-Tasman rival Air New Zealand opted last month for the Boeing planes. It has ordered eight 777Xs in a deal reportedly worth more than $2 billion.

Air New Zealand chief executive, Christopher Luxon, told Reuters last year that the airline was also looking to fly longer routes, such as Auckland to New York and Brazil.

Mr Joyce said Qantas was in talks with pilots about changing a labour contract to increase productivity to help support the business case for an order for the long-haul planes.

Qantas plans to have four service classes on the plane: First, business, premium economy and economy. There will also be a zone for economy and premium economy-class passengers to stretch and hydrate.

Singapore Airlines has only business class and premium economy on the world’s current longest route, from Singapore to New York.

Mr Joyce said Qantas’ success in selling about 90 per cent of economy-class seats on its Perth-London flights showed there was demand for economy class on the even longer Sydney-London route.

The airline launched the “game-changing” 17-hour direct flights just over a year ago.

“There still will be a large economy,” Mr Joyce said.

Qantas is also planning other lengthy routes with the new jets, such as Melbourne-London, Sydney-New York, and possibly flights from the east coast of Australia to other cities in Europe, the US east coast and Brazil, Mr Joyce said.

-with AAP