The Chancellor says no-one “really knows” how much crashing out of the EU will cost the country – undermining repeated ministerial claims that the damage will be minimal and short-term.

Sajid Javid did not dispute his own watchdog’s warning of a £30bn a year hit, saying: “I’ve never pretended that if you leave without a deal it won’t be challenging.”

And he acknowledged the severe impact on businesses, “especially if you a trader with the EU”.

Speaking ahead of his Conservative conference speech, Mr Javid also condemned a call by a former Tory cabinet minister to publish the government’s Brexit proposals as a “ridiculous suggestion”.

The chancellor argued it would put the UK in “absolutely the worst negotiating position”, although ideas have been shared with the EU – and dismissed as insufficient to strike a fresh deal.

And he refused to reveal Boris Johnson’s plans for avoiding the legal commitment to seek a Brexit delay if no deal is passed by 19 October, while arguing: “It’s not about getting around the law.”

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In July, the independent Office for Budget Responsibility (OBR) warned a recession was on the cards after even a relatively benign no-deal Brexit, if it goes ahead on 31 October.

Government debt would rise over the next three years and the budget deficit would widen by around £30bn from 2020-21 onwards, its report said.

Asked if that forecast was “wrong”, Mr Javid – who has refused to publish any further projections – said his focus was on “our mitigation for no deal”.

And he argued all forecasts did not take into account emergency measures to rescue the economy, saying: “No-one really knows, because they couldn’t know, what the economic policy response would be.”

Mr Javid told BBC Radio 4’s Today programme: “What I’m confident about is, with all those mitigations – whether it is the ports or elsewhere – that we will be able to deal with many of the disruptions around no-deal.”

The Chancellor also declined an invitation to describe the allegations of groping made against the prime minister, by the journalist Charlotte Edwardes, as serious.

“I've talked to the Prime Minster about that and, first of all, he couldn't be clearer, absolutely clear, that they are completely untrue and I totally trust him in that,” he said.

Pressed if the party takes the allegations seriously, Mr Javid said: “One thing I have noticed in nine years I've been in parliament, there are personal allegations all the time and each time I've heard about them, whoever they are about, I don't get involved and don't really comment on them because how would I know anything about these allegations?"

On whether crashing out of the EU on Halloween is still possible, despite the Benn Act, he said: “Yes, it is. It's not our preferred outcome, we are working incredibly hard to get a deal by 31 October and I absolutely believe that can still happen.