Kemi Badenoch's flagship party conference policy document overstated the cost of reaching net zero by nearly £500bn, apparently because someone counted the same figures twice on a spreadsheet.

The error appears in the net zero chapter of The Right Way, a 100-page document produced for the Conservative conference in Birmingham. The Tory leader used the phrase repeatedly in her main speech.

The mistake was spotted by the climate publication Carbon Brief, which passed it to the Guardian.

How the number was doubled

Justifying the Conservatives' decision to scrap the legally binding target of reaching net zero emissions by 2050 — introduced under Theresa May — the document says cost estimates "have varied enormously".

It goes on: "In 2020, the Climate Change Committee (CCC) estimated that its route to net zero would cost £957bn." It cites the CCC's Sixth Carbon Budget, saying the report put gross costs at £1.4tn, falling to £957bn once savings were included.

But the figures in the CCC report actually total £478bn — half the number quoted.

The error appears to have arisen because a tally of costs in a CCC spreadsheet also included the row showing the totals for each column, doubling the result.

'We don't recognise this number'

A spokesperson for the CCC, the government's independent climate advisers, said: "We don't recognise this number. The committee has previously offered to meet with Ms Badenoch and talk her through our advice. That offer is, of course, still open."

The committee's most recent analysis puts the cost of achieving net zero by 2050 at about £110bn in total, or around £4bn a year — about 0.2% of GDP.

It is possible the Conservatives took the figure from elsewhere without checking it. The same number appears to have been published by the right-leaning Institute of Economic Affairs think tank and in an article in The Spectator.

The Conservatives and Mrs Badenoch's office were approached for comment.

An engineer's diagnosis

The document is modelled in part on The Right Approach, produced by Margaret Thatcher's Conservatives in 1976, which set out economic ideas later used in the 1979 election campaign. It is intended to provide a precise diagnosis of the UK's problems.

In her foreword, Mrs Badenoch notes that "as an engineer" she is used to diagnosing issues correctly so they can be fixed. Another passage states: "To fix a problem we must first have an accurate understanding of what is causing it."

That emphasis on accuracy makes the error especially awkward for the Tory leader, who has made scrapping the 2050 target a central plank of her pitch to voters worried about energy bills.

The wider argument

The cost of net zero has become one of the sharpest dividing lines in British politics. The Conservatives and Reform UK argue that the target is driving up bills and damaging industry. Labour and the Greens argue that investment in clean energy will cut costs and improve energy security over time.

Economists broadly agree that the upfront investment is large but that much of it is offset by lower fuel spending in later decades. The disagreement is over the balance, timing and who pays.

The episode also lands at a difficult moment for Mrs Badenoch. Robert Jenrick, her former leadership rival, launched a scathing attack on her this week, branding her "Kensington Kemi".

Labour figures were quick to seize on the blunder as evidence that the Conservatives' energy policy rests on shaky foundations.

Supporters of the Tory leader will argue that the case against the 2050 target does not rest on a single figure. Critics will say a document billed as a rigorous diagnosis cannot afford to get its central number wrong by half a trillion pounds.