LONDON • Mr Boris Johnson, the favourite to become British prime minister, has said he is prepared to lead the country out of the European Union without a deal on Oct 31, and that any EU attempt to impose trade tariffs would be akin to a Napoleonic-era blockade.

Britain's three-year Brexit crisis could be about to deepen as Mr Johnson's vow to leave the EU with or without a deal on Halloween could provoke a stand-off with Parliament, which has indicated its opposition to a no-deal exit.

No-deal means there would be no transition period, so the exit would be abrupt, the nightmare scenario for many business leaders and the dream of hardline Brexiters who want a decisive split.

Mr Johnson, a former foreign minister and London mayor, said he is convinced that the EU would agree to a new deal based on parts of outgoing Prime Minister Theresa May's "dead" Withdrawal Agreement.

"My pledge is to come out of the EU at Halloween on 31 October," Mr Johnson, 55, told BBC TV on Monday night, adding that there are "technical fixes" to prevent the return of a hard border between Ireland and Northern Ireland, which is part of the UK.

Mr Johnson reaffirmed his view, contested by many, that Britain could retain tariff-free trade with the EU after a no-deal exit.

Yesterday, he told LBC radio: "I think it would be very bizarre if the EU should decide on their own... if they decided to impose tariffs on goods coming from the UK it would be... a return to Napoleon's continental system."

Napoleon Bonaparte's "continental system" was a blockade whose aim was to cripple Britain's economy during the Napoleonic wars in the early 19th century.

Bank of England governor Mark Carney has said a provision that permits trade to continue unchanged between two parties could only be applied when a trade deal was in place or about to be in place.

Mr Johnson repeated a warning that there would be "creative ambiguity" about when and how a previously agreed £39 billion (S$67 billion) exit bill gets paid to the EU.

He also repeatedly refused to answer questions about an argument with his girlfriend which has raised doubts about his fitness to lead the country.

The EU has refused to renegotiate the Withdrawal Agreement reached with Mrs May last November, and Ireland, an EU member, has indicated it is not willing to change the Irish border "backstop" - a guarantee to ensure no return of hard border checks between it and British-run Northern Ireland.

Mr Johnson also said he did not want a no-deal Brexit - which investors warn would roil financial markets and the European economy - but that it was necessary to put it on the table so that Britain could get the result it wanted.

"The way to get our friends and partners to understand how serious we are is finally, I'm afraid, to abandon the defeatism and negativity that has enfolded us in a great cloud for so long and to prepare confidently and seriously for a WTO or no-deal outcome," he said in the BBC TV interview.

Britain is a member of the World Trade Organisation so tariffs and other terms governing its trade with the EU would be set under WTO rules.

Meanwhile, business leaders have already triggered contingency plans to cope with additional checks on the post-Brexit UK-EU border, which they fear will clog ports, slow trade and dislocate supply chains in Europe and beyond.

Brexit supporters say that in the long term the UK would thrive if cut free from what they cast as a doomed experiment in German-dominated unity that has led to Europe falling behind China and the United States.

Mr Johnson is fighting Foreign Minister Jeremy Hunt for the Conservative Party leadership. The winner will be announced on July 23 and will then replace Mrs May as prime minister.

REUTERS