The UK public should be urged by the government to protect the climate by eating less meat and dairy produce, advisers say.
Cattle are a major source of planet-heating gases, but ministers fear a backlash if they ask people to cut down on steak.
But the Climate Change Committee (CCC) says people should reduce meat-eating for their health, as well as for the planet.
It says the issue's one of many failings of a government which is delivering only a fifth of its pledges on climate change.
People should be asked to eat 20% less meat and dairy produce by 2030, and 35% less by 2050, the CCC insists.
The CCC says Boris Johnson must devise evidence-based policies to encourage healthier diets and set clear targets.
Its report says the PM's "remarkable" climate leadership is undermined by inadequate policies and poor implementation in many areas of policy.
A government spokesman said its net zero strategy, due in the Autumn, would show where carbon cuts would be imposed across the economy.
Net zero refers to reducing emissions of greenhouse gases as much as possible and then balancing out any remaining releases by absorbing an equivalent amount from the atmosphere - by, for example, planting trees.
But the committee complains that the public hasn't been engaged to make changes essential for protecting the climate. In addition to meat and dairy, they are:
Sales of new gas boilers should be stopped by 2033. People will mostly convert to heat pumps instead. This will involve disruption - and the CCC says ministers will have to subsidise the installation cost.
Committee members want to see taxes taken off clean electricity - and maybe shifted on to more polluting gas - although power bills for poor households should not rise.
Frequent fliers will need to be curbed, the CCC believes. Even if low-carbon planes are developed, the UK still cannot let demand for aviation grow unconstrained.
People will need to be consulted over changes ahead - perhaps by groups such as the UK climate assembly.
The report says the government currently lacks policies on these issues and many others. Waste and low-carbon heat networks are said to need policies too.
The committee chairman Lord Deben said the prime minister's commitments on the international stage to cut emissions 78% by 2035 are "remarkable decisions".
He added that the objective of achieving near zero emissions by 2050 sets a major example to other nations.
"The trouble," he said, is that the delivery has not been there. Almost all things that should have happened have either been delayed or not hit the mark. They need to step up very rapidly."
The CCC's chief executive Chris Stark said he was "very concerned by the gulf between promises and actions".
His report laid down some fundamental principles for the journey towards a near zero-carbon economy.
It urges the Treasury to protect the poorest from the cost of climate policies. It says: "The net zero strategy must be underpinned by an approach that distributes the costs, savings and wider benefits of decarbonisation fairly.
"It must encourage action across society, while protecting vulnerable people and companies at risk of adverse impacts."
A government spokesman said: "Any suggestion we have been slow to deliver climate action is widely off the mark. Over the past three decades, we have driven down emissions by 44% - the fastest reduction of any G7 country.
"We have set some of the most ambitious targets in the world for the future.
"In recent months, we've made clear with record investment in wind power, a new UK Emissions Trading Scheme, £5.2bn investment in flood and sea defences, clear plans to decarbonise heavy industry and North Sea oil, and businesses pledging to become net zero by 2050 or earlier.
"Our strategies this year will set out more of the very policies the Climate Change Committee is calling for as we redouble our efforts to end the UK's contribution to climate change."
But environmental group Friends of the Earth said: "The committee's criticisms are spot on. Without a detailed strategy for combating the climate crisis, government promises to decarbonise the economy are simply more hot air.
"With no climate action plan and his government's support for more roads, runways and an overseas gas project, Boris Johnson risks being a laughing stock at the UN climate summit [which the UK is hosting]."
The CCC insists ministers must commit all policies to a "net zero test" to ensure that decisions are compatible with the emissions targets.
But there is a Whitehall logjam of decarbonisation initiatives in the pipeline. They include the Environment Bill and several strategies for different sectors, such as a transport decarbonisation plan and a net zero aviation strategy.
Mr Stark says the environment Department for Environment, Food and Rural Affairs is lagging with policies, and the Ministry of Housing, Communities and Local Government is failing to integrate climate change into the Planning Bill.
All these policies, though, are over-shadowed by the delayed Treasury net zero review, which will determine how much cash is invested into the projected zero-carbon economy.
Some key policies are being delayed by the Treasury, and environmentalists fear that the Chancellor Rishi Sunak may be jockeying for influence with the climate sceptic wing of the Conservative Party by withholding funds needed for the PM's "green revolution".
It is a huge challenge for the Treasury, which will also need to take into account another recent CCC report warning that the nation unprepared for the inevitable impact of a heating climate on the UK.
Follow Roger on Twitter.