PARIS, Oct 20 (Reuters) - French infrastructure group Vinci expects earnings to continue falling in the second half of the year, as weakness at its pandemic-ravaged airports business outweighs signs of a recovery in construction and motorway traffic.

The owner of London's Gatwick airport said on Tuesday the earnings decline should be less steep than in the first six months of 2020, when revenue fell 15% and operating income was down nearly 90%. It expects to return to growth in 2021.

Vinci said revenue fell 6.4% to 12.3 billion euros ($14.5 billion) in the third quarter. It managed to contain some of the fallout from the COVID-19 pandemic after construction projects restarted, and its contracting unit, which operates in areas like energy, picked up steam too.

But its airports business - hit hard by measures to control the pandemic, despite an easing of restrictions in recent months - remained under pressure, including as governments take new steps to contain the virus' spread.

Vinci said it now expected the fall in passenger numbers to reach 70% this year, compared to its previous forecast for a 65% fall.

CEO Xavier Huillard told a conference call that the group, which made a 5.2 billion euro bid earlier this month for Cobra, an industrial unit held by Spanish engineering and infrastructure group ACS's, was now at the due diligence phase of the deal.

($1 = 0.8459 euros) (Reporting by Sarah White; Editing by Mark Potter)