Energy from offshore wind in the UK will be cheaper than electricity from new nuclear power for the first time.
The development, revealed in figures from the government, has been seen as a milestone in the advance of renewable energy.
The plummeting cost of offshore wind energy has caught even its most optimistic supporters by surprise.
Nuclear firms said the UK still needed a mix of low-carbon energy, especially for when wind power was not possible.
The figures, from the Department for Business, Energy and Industrial Strategy, for offshore wind were revealed as the result of an auction for subsidies, in which the lowest bidder wins.
Two firms said they were willing to build offshore wind farms for a subsidy of £57.50 per megawatt hour. That compares with new nuclear plants at a subsidy of £92.50 per megawatt hour for 2022-23.
Emma Pinchbeck from the wind energy trade body Renewable UK told the BBC: “These figures are truly astonishing.
“We still think nuclear can be part of the mix – but our industry has shown how to drive costs down, and now they need to do the same.”
Onshore wind power and solar energy are already both cost-competitive with gas in some places in the UK.
And the price of energy from offshore wind has now halved in less than five years.
Energy analysts said UK government policy helped to lower the costs by nurturing the fledgling industry, then incentivising it to expand – and then demanding firms should bid in auction for their subsidies.
Michael Grubb, professor of energy policy at University College London, called the cost reduction “a huge step forward in the energy revolution”.
“It shows that Britain’s biggest renewable resource – and least politically problematic – is available at reasonable cost.
“It’ll be like the North Sea oil and gas industry: it started off expensive, then as the industry expanded, costs fell. We can expect offshore wind costs to fall more, too,” he said.
The subsidies, paid from a levy on consumer bills, will run for 15 years – unlike nuclear subsidies which run for 35 years.
This adds to the cost advantage offshore wind has now established over new nuclear.
Caroline Lucas, co-leader of the Green Party, said: “This massive price drop for offshore wind is a huge boost for the renewables industry and should be the nail in the coffin for new nuclear.
“The government’s undying commitment to new nuclear risks locking us into sky high prices for years to come. Put simply, this news should be the death knell for Hinkley C nuclear station.”
However, the nuclear industry said that because wind power is intermittent, nuclear energy would still be needed.
Tom Greatrex, chief executive of the Nuclear Industry Association, said: “It doesn’t matter how low the price of offshore wind is. On last year’s figures it only produced electricity for 36% of the time.”
EDF, which is building the Hinkley Point C nuclear plant, said the UK still needed a “diverse, well-balanced” mix of low-carbon energy.
“New nuclear remains competitive for consumers who face extra costs in providing back-up power when the wind doesn’t blow or the sun doesn’t shine,” the French firm said.
“There are also costs of dealing with excess electricity when there is too much wind or sun.”
EDF added that energy from new nuclear plants would become cheaper as the market matures, as has happened with offshore wind.
Eyes will be raised at this suggestion, as nuclear power has already received subsidies since the 1950s but storage of surplus energy from offshore wind is still a challenge.
Prof Grubb estimated the new offshore wind farms would supply about 2% of UK electricity demand, with a net cost to consumers of under £5 per year.
Experts warn that in order to meet the UK’s long term climate goals, additional sources of low-carbon energy will still be needed.