Scott Kirby, chief executive  of United, said: “Hydrogen-electric engines are one of the most promising paths to zero-emission air travel for smaller aircraft, and this investment will keep United out in front on this important emerging technology.”

United may fit the engines to its 50-seater CRJ-550 regional jets, the company said.

ZeroAvia has already signed a deal with Alaska Airlines to put new engines in its fleet of propeller-driven De Havilland DHC-8-400s and with India’s Hindustan Aeronautics to fit them to old and new Dornier 228 aircraft, which typically have 19 seats.

The business also has an agreement with Mitsubishi to work on larger 70-seater regional jets.

Founder Val Miftakhov is banking on using existing planes to get his engines into use quickly. Once certified by aviation authorities, they can radically cut carbon output.

Last month, he said: “It’s not quite as simple as just replacing an old engine with a new engine, of course. But it is not nearly the same high level of effort as the whole new airframe.”

A former Google executive who went on to found an electric car charging start-up, he says a hydrogen-electric powertrain is the best solution. Burning hydrogen in a piston engine or jet engine can produce nitrogen dioxide, or NOx, which is harmful to lung development. 

Instead, using a fuel cell to generate electricity and power a motor means only water and contrails are the byproduct, so long as the hydrogen is made using carbon-free electricity.