The King and the Mayor of London have activated a climate clock which counts down the time left to balance global greenhouse gas emissions to prevent the Earth heating more than 1.5C above pre-industrial levels.

Scientists have said that achieving this is vital to ensuring a safe and liveable planet as even sticking to 1.5C offered just a 50-50 chance of avoiding catastrophic tipping points that would heat the Earth beyond human control.

Almost every country has agreed to meet this target as part of the Paris Agreement in 2015, but experts have warned that after eight years the world is still not on track, with warming of around 2.7C currently predicted by 2100.

The climate clock has a countdown of six years and 24 days, at which point experts say the world will have used up the carbon budget for keeping to the Paris Agreement and the Earth will inevitably heat beyond 1.5C.

Mr Khan, alongside the King on stage at the Climate Innovation Forum in London, activated the clock with a large red button made from plastic washed up on the Gower Peninsula in Wales, which will be recycled into a plant pot and given to the King.

There are 150 versions of the clock around the UK, across London, Birmingham, Glasgow, Manchester, Edinburgh and Southampton.

A dominating image of the countdown will also broadcast in London’s Piccadilly Circus for five days.

Nick Henry, chief executive and founder of Climate Action, said: “We are honoured to be joined by His Majesty King Charles at the Climate Innovation Forum for the national climate clock switch on, during London Climate Action Week.

“This powerful illustration of the scale of the climate emergency also reminds us there is still time to avert disaster.

“We need to align all actors – governments, cities, investors, businesses, and civil society – to move at speed and at scale.

“It is vital that we embrace the pro-growth opportunity of the net-zero transition and turn ambition into transformational action.”

The King also met climate change minister Graham Stuart, who said on stage: “We can be proud of the fact that we have decarbonised more than any other major economy on Earth.

“But it’s not enough, and that’s one of the reasons why we’re funding innovation.

“I’m pleased to announce today that we’ve awarded £80 million to companies developing new clean technologies through our net-zero innovation portfolio.”

The King met some of these innovators, who are creating solutions to environmental problems, such as Futraheat, which works to capture and reuse waste heat from industrial processes, and Arda Biomaterials, which turns feedstocks into materials for fashion, home goods and other industries.

Pascal Soriot, chief executive of AstraZeneca, said on stage: “This is not only a crisis that will happen in 20 or 30 years, this is a crisis that is here today.

“The pandemic globally cost seven million lives. And, of course, it’s an awfully large number of people dying from Covid.

“But actually, pollution and climate change cost us seven to nine million lives every year.

“And some people would think OK, well this is something that is happening in faraway countries due to flooding, drought, extreme temperatures, but it is actually here, it’s affecting us all.”