The UK will miss its legally binding carbon targets without urgent government action, official advisers have warned.

The Committee on Climate Change (CCC) said vague ambitions, such as banning new petrol and diesel cars by 2040, must be turned into solid plans and new policies, such as planting more trees, are needed.

The CCC also warned that there are “significant” risks to the delivery of existing projects, such as the new Hinkley Point nuclear plant. However, the CCC also praised Theresa May’s government for putting green growth at the centre of the nation’s economic strategy for the first time when it published its Clean Growth Strategy (CGS) in October.

“There has been a very fundamental change of stance,” said Lord Deben, chair of the CCC. “But that, of course, doesn’t mean there is not a very great deal more to do. There is a real pressure and urgency.”

Deben said the CCC analysis released on Wednesday was the best-case scenario and had assumed ministers will deliver all the carbon cuts pledged so far. “We have given every benefit of the doubt,” he said. “But even if they do all the things they say they are going to do, to the maximum, there will still be a gap.”

The CCC’s independent assessment of the CGS analyses whether it puts the UK on course to meet its legally mandated carbon cuts in 2025 and 2030, and charts the most cost-effective path to an 80% reduction in emissions by 2050. The UK has already cut greenhouse gas emissions by 42% since 1990 and is close to phasing out coal, but it is not on track to meet the future targets, the CCC found.

The CCC highlighted a series of government pledges that have little or no detail on how they will be delivered, including giving all homes a good level of energy efficiency by 2035. Deben also attacked large housebuilding companies for not doing enough.

“If Persimmon had spent the £110m bonus it has given to its chief executive [Jeff Fairburn] on the 18,000 houses it has built this last year, it could have saved very significantly the energy bills of everybody who bought a house and contributed significantly to emissions reduction,” he said.

Also highlighted was the critical need to roll out carbon capture and storage plants (CCS) by the 2030s, to trap and bury emissions from fossil fuel plants. “We cannot underline more strongly that without CCS, meeting our statutory targets will cost a great deal more,” Deben said. “CCS is essential.”

The UK’s North Sea is seen as a perfect place for CCS development but former chancellor George Osborne killed a £1bn programme in 2015. Since then only £100m has been pledged in its place, a sum “not commensurate with its importance” according to the CCC.

But Deben also said oil and gas companies have not stepped up to develop CCS: “If you want to argue [against fossil fuel divestment campaigns], you have got to show you are taking really seriously the move away from fossil fuels. It is very dangerous for the industry not to take this very much more seriously.”

Gaps in government policies identified by the CCC include a clear plan to drive up sales of electric cars to the upper end of its goal of 40-70% of all cars by 2030 and to approach 100% by 2035. Another is the need to plant 70,000 hectares of trees by 2025 – it would take a century to achieve that at current rates.

The CCC also warned that relying on new nuclear plants for low-carbon electricity carried risks. “Nuclear has a history of not delivering on time or on budget,” said Deben. Any new nuclear plants after Hinkley “would have to come in at a much, much reduced cost”, given the very low cost offshore wind now offers, CCC officials said.

A spokeswoman for the Department of Business, Energy and Industrial Strategy said: “The UK has reduced emissions per person faster than any other G7 nation and our CGS is the next ambitious milestone. But we have always said it is only the start of a process.”