LONDON (Bloomberg) - Prime Minister Theresa May won lawmakers' backing to trigger the start of Britain's exit from the European Union by the end of March next year after promising to give Parliament the chance to scrutinise her plan first.

After more than six hours of debate, the House of Commons on Wednesday (Dec 7) voted by 448 to 75 in favour of a motion that commits lawmakers to support her timetable for formally notifying the EU that Britain is leaving.

The motion also ties May to setting out her negotiating plan so lawmakers can examine the details before she begins the Brexit process.

The exit talks will be "the most important and complex negotiations in modern times," Brexit Secretary David Davis told Parliament in London on Wednesday, as he warned that the UK might not get what it wants in Brussels.

"This is a negotiation; it's not a policy statement, and therefore where we're aiming for may not be the exact place we end up."

Wednesday's vote is not legally binding on either the government or members of parliament but it is a politically significant moment for the UK's complex efforts to withdraw from the EU following June's referendum.

May has been unwilling to give details of her negotiating strategy, saying it would give an advantage to Brussels before the start of talks next year.

Wednesday's motion means she will have to set out in more detail the shape of the final deal she wants to see before triggering the Brexit process under Article 50 of the Lisbon Treaty. During the debate, Davis began to put flesh on the bones of the government's aims.

The UK could seek to model its new relationship on trade rules used by Switzerland or Norway, Davis said. The government is considering at least four different options for Britain's future relationship with the customs union, which imposes common tariffs on external countries while allowing EU members tariff-free trade with each other, he told lawmakers.

"There are several options of customs union; one is shown by Norway, which is in the single market but not in the customs union," Davis said.

"You've got one shown by Switzerland, which is neither in the customs union nor in the single market but has a customs agreement, so there are a whole series of options that exist."

Red, White and Blue

May has insisted she is not seeking to replicate another country's relationship with the EU but instead wants a patriotic "red, white and blue Brexit" that works for the UK, referring to the colors of the British flag. The premier says she wants the UK to strike new trade deals with other countries outside the EU, leading the world into a new free-trade era.

Still, officials say this cannot happen until the UK leaves the European customs union.

Davis was challenged by members of his own ruling Conservative Party, as well as opposition lawmakers, to set out whether the UK will remain inside or outside the customs union.

The UK doesn't face a "binary" choice, the Brexit secretary said. "There are about four different possibilities and we are still assessing that," he added.

The difficulty in reaching a consensus in Parliament was underlined by rank-and-file lawmakers in May's Conservative Party. Former Work and Pensions Secretary Iain Duncan Smith, a prominent campaigner for Brexit, said it would be better to remain in the EU than to leave the bloc while staying in the customs union. Former Chancellor of the Exchequer Kenneth Clarke, the only Conservative to vote against the motion, said the June referendum wasn't a vote on the issue.

Mystery to Most

"Our discussion on whether we should be in the customs union, and the consequences one way or another were not decided by the referendum," Clarke said.

"These choices, which the ministers are now struggling with, for which they should be accountable to us, would have been a mystery to 99 per cent of the people listening to the debate and voting in the referendum."

Davis used the debate to challenge opponents to back a plan to trigger the start of Brexit by the end of March. It was the first time lawmakers have had the chance to vote on the timetable.

In an amendment published the night before the debate, May promised to set out her plan for Brexit and to allow lawmakers to scrutinize it before she triggers Article 50. Britons voted by 52 per cent to 48 per cent in June to quit the bloc and Davis said it's "inconceivable" that lawmakers will not get a vote on whether to accept the final deal once negotiations have finished.

Vague Plan?

Opening the main debate, the opposition Labour Party's Brexit spokesman, Keir Starmer, said he does not want to "frustrate" the process "or to delay the timetable," but he called on the government to produce a minimum plan for what it wants to achieve in talks with the EU.

"We're not going to have a situation where the government seeks a vote in a vacuum or produces a late, vague plan," Starmer told lawmakers.

May's team must set out a strategy that includes "enough detail and clarity to end the circus of uncertainty" on the UK's membership of the EU single market and customs union, Starmer said.

Labour lawmaker Stephen Kinnock said the motion will give the opposition an opportunity to scrutinize May's approach to the talks. While his party recognizes the result of the referendum, it has a duty to ensure Britain gets the best possible deal, he said.

"We should be holding the country to account but not to ransom," Kinnock told BBC TV. "I'm keen that we leave on the best possible terms and it's my duty as an opposition MP to hold the government's feet to the fire."