A Liberal Democrat government would revoke Article 50 and stop Brexit “on day one”, leader Jo Swinson is to vow.
In her first speech to the party’s conference as leader, Ms Swinson will reiterate its policy of overturning the law ensuring the UK will leave the EU.
A no-deal Brexit would be like “burning your house down”, Ms Swinson will add.
The East Dunbartonshire MP will also appeal to Scottish voters to reject the SNP and join a “bigger movement for change” across UK to stop Brexit.
Ahead of an expected general election, Ms Swinson will present herself as a candidate for prime minister, saying she cannot wait to take on Boris Johnson, Labour’s Jeremy Corbyn and Brexit Party leader Nigel Farage.
She has ruled out entering into a coalition with the Conservatives or Labour if a general election delivers a hung Parliament, saying neither Mr Johnson nor Mr Corbyn were “fit to be our prime minister”.
On Tuesday, as the Supreme Court began hearing two appeals relating to the suspension of Parliament, Lib Dem conference delegates backed an emergency motion calling for the suspension of Parliament to be reversed.
Lord Newby, Lib Dem leader in the House of Lords, said the prime minister had prorogued Parliament on “a transparent lie” in a way that had reduced the Queen to “a ceremonial rubber stamp”.
Earlier at the conference Lib Dem members voted overwhelmingly to back her proposal for a manifesto pledge to revoke Article 50 if the party came into power with a majority government.
‘No good Brexit’
In her speech, Ms Swinson will criticise Mr Johnson’s pledge to take the UK out of the EU by the 31 October, with or without a deal.
She will tell the conference in Bournemouth the prime minister’s spending on no-deal preparations is “sickening”.
“The truth is you can’t plan for no deal. Planning for no deal is like planning to burn your house down,” she is expected to say. You might have insurance, but you’re still going to lose all your stuff.”
Ms Swinson, who succeeded Sir Vince Cable as Lib Dem leader in July, is expected to add: “The first task is clear. We must stop Brexit. There is no Brexit that will be good for our country.
“Brexit will put lives at risk. Brexit will hurt our economy.”
Ms Swinson will also criticise Mr Johnson for withdrawing the Conservative whip from 21 Tory rebels – including one, Sam Gyimah, who later joined the Lib Dems – and the prime minister’s decision to suspend Parliament.
She will say Mr Johnson is “silencing critics, purging opponents, ignoring the law”.
Ms Swinson is also expected to say Mr Corbyn is holding on to Eurosceptic views.
“Even now, when faced with all the clear and obvious dangers that Brexit brings, Jeremy Corbyn still insists that if Labour win a general election, they will negotiate their own Brexit deal to take us out of the EU,” she will say.
But the shadow attorney general Shami Chakrabarti described the Lib Dem’s promise to revoke Article 50 without a further referendum as “illiberal and anti-democratic”.
Turning to Scotland, Ms Swinson will highlight its support to remain in the EU in the 2016 referendum.
“Together we can stop Brexit,” she will say. “We are building a movement across the United Kingdom that is on the verge of stopping it.
“A big vote for the Liberal Democrats in Scotland at the general election will give us the final push that we need.”