Children not yet born “will not forgive” today’s world leaders if they fail to confront the climate emergency, Boris Johnson has told the Cop26 summit.

Opening the landmark event in Glasgow, the prime minister pointed out the young had a far greater stake in the looming calamity than the people in the room – whose average age was “over 60”.

“The people who will judge us are children not yet born – and their children,” Mr Johnson said, adding: “We mustn’t fluff our lines or miss our cue.

“If we fail, they will not forgive us. They will judge us with a bitterness and a resentment that eclipses any of the climate activist of today. And they will be right.”

The appeal came as the prime minister likened the climate crisis to a James Bond movie – except the “tragedy is that this is not a movie and the doomsday device is real”.

The summit opens against the gloomy backdrop of the G20 group of the biggest economies failing to make fresh climate commitments after their two-day gathering in Rome.

A final communique agreed merely to “enhance when necessary” plans to cut carbon emissions by 2030 – the cut-off point for averting disaster, scientists say.

Far from agreeing to “consign coal to history” – the UK’s aim for Cop26 – it sets no date for phasing out the fossil fuel, which will happen only “as soon as possible”.

And the G20 spurned the UK’s call to commit to net zero emissions by 2050 – but no date was set, because big emitters China, India and Russia have rejected it

Mr Johnson told the opening ceremony of the terrible consequences of failing to limit the global temperature rise since industrialisation to 1.5°C – something he has admitted cannot be achieved in Glasgow.

“Two degrees more and we jeopardise the food supply for hundreds of millions of people, as crops wither, locusts swarm.

“Three degrees and you can add more wildfires and cyclones – twice as many of them, five times as many droughts and 36 times as many heatwaves.

“Four degrees and we say goodbye to whole cities – Miami, Alexandria, Shanghai – all lost beneath the waves.

“And the longer we fail to act, the worse it gets and the higher the price when we are eventually forced by catastrophe to act.”

The prime minister pleaded: “If we don’t get serious about climate change today, it will be too late for our children to do so tomorrow.”

But Rachel Reeves, Labour’s shadow chancellor, accused Mr Johnson of hypocrisy, after a budget that failed to fund the green transition while making flying and driving cheaper.

“The PM says it’s one minute to midnight on climate, but he and the chancellor act like its 2.15 in the afternoon,” she tweeted.

“The chancellor ignores the huge costs of not acting and won’t use green jobs to grow our economy – while the PM is all talk and no action.”