Boris Johnson has been urged to get the UK back on track with tackling the emissions heating the planet.
The Committee on Climate Change, the UK's official climate watchdog, says the government needs to be meeting its own targets to have credibility with other nations.
But it warns that UK efforts to address the climate crisis have so far fallen short.
The comments come ahead of a vital global climate conference next November hosted by the prime minister in Glasgow.
A letter from the climate committee's chairman, Conservative peer Lord Deben, says urgent action is needed in five areas:
An ambitious, well-funded strategy for entirely removing fossil fuels from the UK's building stock - that will mean much better insulation and switching gas boilers for cleaner alternatives.
An early consultation on phasing out petrol and diesel cars by 2030 - the current date is 2040.
Delivering on the Conservatives' manifesto commitment for 40GW of offshore wind by 2030 - that will mean huge expansion of wind farms offshore.
Working out how to fund emissions reductions from industry and how to pay for an infrastructure for hydrogen heating and equipment to capture emissions from heavy polluters.
Land use and agriculture
Introducing a world-leading package through the Agriculture and Environment Bill to cut emissions from farming and pay for the 30,000 hectares (75,000 acres) of annual tree planting promised in the manifesto.
The Climate Change Committee says that most of all, the prime minister should make it clear that he is putting the government's target for net zero emissions by 2050 at the heart of the UK's economic strategy.
And urgent action is also needed on adapting to climate change in the UK itself.
Lord Deben said: "We are worryingly unprepared for the changes ahead. Many departmental plans do not even include a basic assessment of climate risk."
He claimed the government could increase flood defence spending by £4bn and stop people paving over open ground in cities.
Ministers should find ways to stop people over-heating in homes, workplaces and public buildings, and they should also protect the natural environment by increasing tree planting and restoring peatland.
Boris Johnson has previously said he wants the UK to be a world leader in climate change.
Mostly when he speaks on the subject he promotes the notion that technology can solve climate problems.
Environmentalists celebrate the role of technology too - but warn that lifestyle changes will still be needed.
In a previous paper, the Climate Change Committee assumed that some people would have to cut down on meat as a contribution to meeting emissions.
Transport academics have also said that Conservative plans for road building - and possibly airport expansion - would both increase emissions.
Follow Roger on Twitter @rharrabin.