Jeremy Hunt has vowed to “cease all discussions” with Brussels at the end of September if it has failed to budge on Brexit and let the UK crash out of the EU a month later.

In a dramatic ramping up of his Brexit threats, the foreign secretary pledged a comprehensive no-deal plan by the end of August, with all civil service leave cancelled.

And he said, of the new 30 September deadline: “If my judgement is that there is no deal to be done, I will immediately cease all discussions with the European Union and focus the whole country’s attention on no deal preparations.”

The move is a clear attempt to outflank Boris Johnson, who remains the clear favourite to win the Tory leadership race after his vow to leave the EU on Halloween “deal or no deal”.

Mr Hunt said he would allow only three weeks for fresh talks with the European Commission after his no-deal Brexit plan was published at the end of August.

And he said: “If there is no engagement on this deal, if it is apparent that the Commission is simply not interested in negotiating, if there is no willingness to tackle the shortcomings of backstop then there will be no kicking the can down the road and we will intensify and finalise our preparations to leave without a deal.

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“So from the start of my premiership, I will work on the basis we are leaving on 31 October without a deal unless the Commission changes its position.”

Giving a speech in London, Mr Hunt denied his new strategy was a “take it or leave it” threat to the EU, insisting talks could take place during late July and August.

However, there will be no new Commission in place until the autumn – and the EU has repeatedly insisted the divorce deal, including the flashpoint Irish backstop issue, cannot be renegotiated.

There is not even a European Council planned until the middle of October, at which EU leaders would be able to sign off on any changes, even if they did back down.

Mr Hunt said he would also order the Treasury to have a “no-deal Brexit Budget” ready for delivery in the first week of September – something Mr Johnson is also planning.

He confirmed a no-deal spending splurge worth almost £20bn, including compensating fishermen and farmers for having to pay new tariffs at a cost of £6bn.

Corporation tax would be slashed to just 12.5 per cent, at an estimated cost of £13 billion a year, regardless of whether the UK leaves the EU without an agreement.

Fresh criticism came from Philip Hammond, a chancellor certain to be sacked when Theresa May leaves on 24 July, who pointed out his “war chest” would be needed to mitigate a crash-out Brexit.

“The “fiscal firepower” we have built up in case of a no-deal Brexit will only be available for extra spending if we leave with an orderly transition,” Mr Hammond tweeted.