The man in charge of Britain's economy got the cost of a high-speed railway wrong by at least £20bn in a radio interview.

Philip Hammond told BBC Radio 4's Today programme that HS2 will cost the taxpayer £32bn, when in fact it is expected to cost more than £52bn.

The Chancellor had been attacking Labour's policy costings, which he claimed "don't add up", when he made the ironic gaffe.

Social media users pointed out the mistake, which happened yesterday, was not immediately reported by most major news outlets, with some commenters suggesting the Conservative Party had a "free pass" with segments of the press.

In contrast, shadow Home Secretary Diane Abbott was vilified by journalists earlier this month when she got confused about the cost of recruiting 10,000 new police officers live on LBC radio. 

Ms Abbott gave varying estimates for the cost of the new recruits, ranging from £300,000 to £80m — both of which host Nick Ferrari said did not make sense.

However some people said Ms Abbot's poor sums paled in comparison to Mr Hammond's, especially given that he has been in charge of Tory spending for almost a year already.

“Just answer this question,” asked John Humphrys, who presents the flagship news and current affairs programme, on Wednesday morning. “How much is HS2 costing?

"Thirty-two billion?" Mr Hammond replied. "About 32 billion pounds."

Mr Humphrys responded: "Thirty-two? Not 52 billion?"

"There's a huge amount of contingency built into the funding for these projects," Mr Hammond argued.

"Indeed it is and it is usually met, that contingency, isn't it?" retorted Mr Humphrys. "Because almost always things cost more than we expect."

Without addressing his mistake directly, Mr Hammond tried to justify the cost of building the 250mph rail line linking London, the West Midlands, Leeds and Manchester 'as spread over 15 years as part of our infrastructure investment plan'.

It is possible Mr Hammond got confused about the cost of HS2 because it has been revised upwards several times since the controversial plan was first announced.

In 2015, an analysis by The Daily Telegraph found HS2 to be the most expensive high speed project in existence. The price estimate at the time of £42.6bn put it at more than ten times the cost per kilometre of some global counterparts — the estimated price tag has since risen by £10bn. 

Jeremy Corbyn has said the Labour party will re-nationalise the railways "line by line" with ticket prices expected to fall as a result.

But Labour was criticised after the release of its manifesto earlier this week for providing detailed costings for all of its policies except for its re-nationalisation plans — which could run into the tens of billions of pounds. 

UK news in pictures

Show all 51

Under the party's own rules, it has to balance day-to-day spending and aim for an operational surplus by 2022, but it can borrow for infrastructure spending.

The leadership argue the one off costs of buying back assets like the railways and postal service will be recouped in the long run. 

Mr Humphrys shot Mr Hammond down for saying the rival party's sums "did not add up".

The presenter said: "I understand that that is the mantra, that they 'don't add up', but the fact is when you look at it they [Labour] have covered the running costs, the things people will really like to see a bit of money spent on like scrapping tuition fees, investment in schools, NHS, reversing benefit cuts and so on. The sort of things people will say yes I want to see that happening. And they have covered that, they have told us how they are going to pay for that."

Mr Hammond's mistake came on the same day that Theresa May refused to say if he would be kept on as Chancellor if the Tories are re-elected next month.