Opening a new coal mine in west Cumbria would be "absolutely indefensible", according to the Climate Change Committee's chairman Lord Deben.
A decision on whether to grant planning permission for the colliery near Whitehaven is due next month.
The committee is an independent body which advises the government on emissions targets.
Environmentalists have warned that demand for coking coal for steel is declining, the coal from the Cumbrian mine would largely not be used in the UK steel industry and it would not replace imported Russian coal.
Speaking as the independent advisory Climate Change Committee launched its annual report on the UK's progress in tackling greenhouse gases, Lord Deben said the invasion of Ukraine meant the UK should be making decisions to protect energy supplies.
That is what the Government had done with a focus on renewables and nuclear, he said.
But he said: "As far as the coal mine in Cumbria is concerned, let's be absolutely clear, it is absolutely indefensible.
"First of all, 80% of what it produces will be exported, so it is not something largely for internal consumption.
"It is not going to contribute anything to our domestic needs in the terms we're talking about, the cost of energy and the rest."
He said the mine's backers claimed it would provide coal for a particular industry which is currently imported from elsewhere but did not mean it would reduce the amount of coal being produced - largely in America.
And he warned it would damage the UK's leadership on climate change, saying:
"All it means is that we create another example of Britain saying one thing and doing another. We do not need this coal mine."
But he said the Government should find a way of ensuring the jobs that might have been created by the mine be replaced by alternative green jobs.
Lord Deben said he was not worried about the move to keep a coal-fired power plant open longer than planned this winter to shore up energy supplies, despite a commitment to end UK coal power by October 2024 - as long as a very close eye was kept on it.
"We are under serious threat because of the invasion of Ukraine by the Russians, and in those circumstances the Government has to make sure that we keep the lights on.
"So I don't find it unacceptable to say for this moment, given that we've got this (power plant), this is not the moment actually to make it impossible to use it. I hope we won't have to use it."
And it would not get in the way of the fact that investment in renewables and nuclear would make coal plants no longer necessary, he suggested.
Friends of the Earth energy campaigner, Tony Bosworth, said: "Lord Deben is right - the case against a new coal mine is overwhelming in climate terms. And the market for its coal is declining before it even opens. Saying no to this mine should an open-and-shut case for a government serious about the climate crisis.
"Instead, it should invest in green jobs in West Cumbria, developing our huge renewable resources and insulating people's homes to cut energy bills."
The government are not commenting today on this issue - it will be up to the Communities Secretary Michael Gove to decide whether to approve or block the mine
His deadline is the 7th of July.