LONDON - No amount of planning can prepare Britain for the disruption of leaving the European Union without a deal, Jo Swinson, leader of the anti-Brexit Liberal Democrat party will say on Tuesday.

As the clock ticks down to Britain's scheduled exit from the EU on Oct. 31, Prime Minister Boris Johnson has yet to agree a deal with Brussels that would protect the flow of goods and services to and from its largest trading partner.

That has raised the prospect of an unmanaged 'no-deal' exit - something Johnson says his government is rapidly preparing for, but which opponents of Brexit say will wreak havoc on Britain's $2.8 trillion economy.

Swinson has already used her party's annual conference to toughen their anti-Brexit stance, promising to cancel Brexit if the Liberal Democrats score an unlikely and unprecedented victory in a widely expected early election.

On Tuesday she will seek to burnish those credentials by attacking Johnson's Brexit strategy as "sickening" and warn voters not to believe him when he says a no-deal Brexit can be managed and planned for.

"The truth is you can’t plan for no-deal," Swinson will say on the final day of the conference in Bournemouth, on England's south coast.

"Planning for no-deal is like planning to burn your house down. You might have insurance, but you’re still going to lose all your stuff."

The government has acknowledged there will be "bumps in the road" if it cannot agree an exit deal, but says it has plans in place to mitigate possible food, fuel, and medicine shortages.

As a party which has never won more than 62 seats at an election, the prospect of Swinson forming a government is a long way off, even though British politics is at its most unpredictable juncture in decades.

Two different polls on Sunday put them 21 percentage points and 8 percentage points behind the Conservatives, and also trailing the opposition Labour Party.

Nevertheless, by reshaping their liberal, centrist organisation into a party at one extreme of the Brexit spectrum they are hoping to cash in on the anger of the 16 million voters who backed 'remain' in the 2016 referendum on leaving the EU.

Swinson will say her ambition for the party, which currently holds 18 seats in Britain's 650-seat parliament, knows no limits, and ask voters to consider her as a candidate for prime minister.

"The tired old parties have failed, looking inward at a time of national crisis," She will say. "Our country needs us, at this precarious time. We do not have ten, or fifteen years. We need to seize the opportunity now."

(Reporting by William James; Editing by Giles Elgood) ((william.james@thomsonreuters.com; +44 207 542 3374; Reuters Messaging: william.james.thomsonreuters.com@reuters.net))