The Prince of Wales arrived on an electric bus for an event about tackling climate change and protecting the environment, which was held at London's Guildhall.

On a meltingly hot day, Prince William told delegates, including London mayor Sir Sadiq Khan, about the importance of helping to "repair" the planet.

Prince William was marking the impact of his Earthshot Prize, saying the ideas for solving environmental problems made him feel "more optimistic than ever".

But he said that when we have the evidence for what works in protecting the environment, "history will ask what we did with it".

The Earthshot Prize is a 10-year scheme launched in 2020 to find innovative ways of saving the planet, with £1m in funding for five winners each year.

Now the halfway mark has passed, Prince William told his audience there had been tangible achievements from the award-winning projects, including the equivalent of taking four million cars off the road, by cutting 18 million tonnes in emissions.

There had been 85 million hectares of land, forests and rivers which had been "protected, restored and improved".

Future schemes would include improving battery life for green energy storage, developing farming techniques in India that would be resilient to a changing climate, and eco-friendly dyes and colours for the fashion industry.

Speakers hailed the achievement of Khan's administration in London in showing how a major city could reduce vehicle emissions.

The mayor told the meeting, part of London Climate Action Week, about the damaging health impacts of air pollution.

And Khan praised Prince William's role, saying his involvement was "authentic... he really believes in this stuff".

Robert Irwin, conservationist and TV presenter, spoke of the need for both "urgency and optimism" in seeking to protect the environment.

The Earthshot Prize's chief executive, Jason Knauf, said that optimism might seem "jarring" when there was such division in the political arena about climate challenges.

But he said that "despair and defeatism are mindsets we cannot afford".

Prince William said the "challenge can still feel immense", but there were ideas for what could tackle it.

"One day people will look back at this decade and ask: when the evidence was clear, what did we do with it?" William asked his audience.

"When we saw forests being restored, did we help restoration spread? When we saw clean air policies working, did we help other cities follow?"