Theresa May has called on Prime Minister Rishi Sunak to “press the accelerator” on the transition to net zero and “not roll backwards”.

The Conservative former prime minister said the “best long-term decision” the Government can make is about climate change as it underpins the “long-term future” of the United Kingdom.

After accusing ministers of not being sufficiently “strong in ambition” to meet the 2050 net zero target, she added: “It’s no good waking up on January 1 2045 and saying we’ve got five years to do something.”

Her comments came as the Commons debated the King’s Speech, which contained plans to grant new North Sea oil and gas licences every year.

In a subtle rebuke to Mr Sunak, Mrs May also reminded him it was her government that legislated a net zero emissions target by 2050.

She said: “I welcome the long-term ambition that the Government has expressed, its need to take long-term decisions.

“Because good government is not about grabbing short-term decisions to get a headline, it is about doing what is in the national interest and in the longer-term future interest of this country.

“But on that point, I was rather surprised recently when I received an email in the name of the Prime Minister sent out I think by the Conservative Party, where it said the following: from net zero to HS2, smoking to education, we are going to tackle the challenges that other politicians have been afraid to even talk about.

“Now since I read that I’ve been racking my brains as to which prime minister it was (that) put net zero in 2050 into legislation? Answers on a postcard please.”

She went on: “I think in relation to the King’s Speech, and the Government’s programme on climate change and environmental degradation, the Government is missing an opportunity.

“What we need to do now is press the accelerator on the transition to a green economy not try to draw back, and I fear that despite the fact that the King’s Speech says ministers will seek to attract record levels of investment in renewable energy sources, that that is not sufficiently strong in ambition from the Government to make sure that they are making that transition quickly enough to ensure that we reach net zero in 2050.

“It’s no good waking up on January 1 2045 and saying we’ve got five years to do something. Let’s do it now because that will be even more costly for members of the public.”

Mrs May said she was worried about the Government “giving some mixed messages to investors”.

She told the Commons: “They need to have the confidence to invest in our transition to a green economy and we need to show that the Government is pressing the accelerator on that, because the best long-term decision that we can make is about climate change because the long-term future of this country and of the people of this country depend on us dealing with climate change and environmental degradation.

“So I want the Government to press the accelerator, not to roll backwards.”

In relation to other “elements” she would have liked to have seen in the King’s Speech, Mrs May pointed to plans to reform the Modern Slavery Act and in particular to “introduce stronger measures in relation to businesses and their supply chains”.

She also criticised a “lack of commitment to a new Mental Health Act”.