Boris Johnson has attacked the £18bn cost of Britain’s first nuclear power station in two decades as “a disgrace” – just one month after David Cameron announced the deal and hailed it as a “flagship project of cooperation” between China and the UK.

In a surprisingly scathing criticism of the Government, Mr Johnson – who attends Mr Cameron’s political cabinet meetings and is George Osborne’s main rival to be next Tory leader – said their pledge to underwrite the deal with £2bn of taxpayers’ money was an “extraordinary amount of money to spend”.

Work on the Hinkley Point C in Somerset is set to begin within weeks after Mr Cameron announced that a deal had been struck between French firm EDF and state-owned China General Nuclear Power (CGN) in October.

China pledged £6bn investment – a third of the total cost, with EDF funding the remaining £12bn, while the Government has agreed a “strike price” – a guaranteed price paid for electricity generated by Hinkley Point of £92.50 per megawatt hour for 35 years.

However the huge cost of the plant will ultimately be paid for by consumers through their bills.

Asked by Baroness Jones, a Green party London Assembly member who is fiercely opposed to nuclear power, whether he supported the building of Hinkley Point C despite its cost, Mr Johnson said: “I’m totally with you on that one – it’s a disgrace.

“Do I think the deal on nuclear power looks like good value for money? It looks like an extraordinary amount of money to spend."

“But I think we have been left in a very difficult position by previous Labour administrations with our energy supply; we need to have security of supply – nuclear has got to be part of the mix – it won’t be the whole solution but it has to be part of it and in an ideal world we would still be one of the world’s great [nuclear innovators] if we had more nuclear physicists than France or Korea.