Robert Irwin and Prince William have reunited to jump onto public transport in London, but not the red buses the city is famous for.

Dressed in a stylish khaki outfit, the 22-year-old wildlife warrior joined the prince on an electric green bus on Tuesday (local time).

The pair, who have a burgeoning bromance, were promoting their shared passion for the environment as part of the prince’s Earthshot Prize.

They attended an event at London’s Guildhall to honour winners and finalists and promote the event’s green achievements.

William travelled to the event on London’s 3000th electric bus with the Aussie conservationist and TV presenter.

Irwin is a global ambassador for the prince’s Earthshot Prize.

“Whilst onboard they shared an inspiring conversation on the progress taking place to protect our planet,” an Earthshot press release said.

The Earthshot Prize Impact Assembly is part of London Climate Action Week.

Irwin told HELLO! magazine that it was “always wonderful catching up with Prince William”.

“The last time I saw him, he Facetimed me during Dancing with the Stars, to wish me well on that. I got to represent conservation to a stage I never thought I would be doing. So it was great.”

Irwin said the Prince of Wales had an “authentic passion” and he “really cares” about the world.

“He cares very, very deeply, and you know someone with his platform that is just mobilised, creating as much change as possible, is fantastic,” he said.

“He’s been doing this for a very long time, and I really think that he’s seen firsthand what it is that the world needs, and I think it’s just fantastic that he uses his platform to unite people.”

Commenting on London’s shocking heatwave, Irwin told HELLO!: “I mean, it’s a reminder of what we’re here for, isn’t it?”