By Kylie MacLellan and William James
LONDON (Reuters) - Britain faces a chaotic exit from the European Union if lawmakers vote against legislation designed to sever political, financial and legal ties with the bloc, Britain's Brexit minister David Davis said before a parliamentary vote.
Parliament is due to hold a late-night vote on whether to let the central plank of Britain's Brexit plan -- the EU withdrawal bill -- move to the next stage of the law-making process.
The government is expected to win the vote, but it is the most serious test yet of Prime Minister Theresa May's leadership after she lost her parliamentary majority at a June 8 election and failed to win a clear mandate for her Brexit strategy.
The bill seeks largely to copy and paste EU law into British legislation to ensure Britain has functioning laws and the same regulatory framework as the bloc at the moment of Brexit, something the government says provides certainty for companies. [nL8N1LO2DF]
"A vote against this bill is a vote for a chaotic exit from the European Union," Davis said in a statement.
"Businesses and individuals need reassurance that there will be no unexpected changes to our laws after exit day and that is exactly what the repeal bill provides. Without it, we would be approaching a cliff edge of uncertainty which is not in the interest of anyone."
Failure to clear the first of many hurdles in the law-making process would present a major problem for May, who now relies on the support of a small Northern Irish political party to govern and faces persistent questions about her future as leader.
"She needs to keep going, get this thing done...we need to get this great ship launched," Foreign Secretary Boris Johnson, a one-time rival for the leadership of the party and a prominent pro-Brexit campaigner, told the BBC.
With the support of Northern Ireland's Democratic Unionist Party, May has a working majority of 13 in the 650-seat parliament.
Most potential pro-EU rebels within May's Conservative Party have signalled they are willing to back the plan at this stage, although they are expected to raise objections later in the legislative process when more detailed scrutiny takes place.
The opposition Labour Party has said it plans to vote against the bill unless the government comes forward with concessions, and has said several clauses in the legislation amount to a "power grab" by government.
But some of its lawmakers said they had to pass the bill at this vote to meet their voters' demands.
Caroline Flint, a Labour lawmaker for a northern English constituency which voted to leave the EU, told parliament: "The truth is whoever was in government we would have to pass a bill of this kind to prepare for leaving the EU in March 2019."
"There can be little disagreement on that, unless your ambition is to thwart the result of the EU referendum and prevent or delay the UK leaving the EU."
The government has promised concerned lawmakers that ministers would not use the wide-ranging powers to make "substantive changes" to law and some have said they will seek changes to the bill at later stages.
To vote down the bill, Labour would need to convince EU supporters in the Conservatives to side with them, but some more vocal pro-EU Conservative lawmakers have said they will vote with the government.
Pro-EU Conservative lawmaker Ken Clarke told Sky News he expected the government to have to change the wording of the bill to win over parliament.
"We have been reassured they are not going to use these powers in any policy-making way ... parliament would be sensible to get them to write it so they are not giving themselves the possibility of using powers that no government has ever tried to take at the expense of parliament before," he said.
The vote is the first of many stages the bill must pass before it becomes law. Next, it will face line-by-line scrutiny and, if approved, then move to the unelected upper house of parliament - where May has no majority - for a similar process.
The process is expected to take months to complete and both houses should agree the final wording before it can be passed.
(Editing by Guy Faulconbridge and Andrew Heavens)