Brexit Bill in UK parliament; passage likely to be smooth

British MPs debated until midnight on Tuesday over the merits of a bill giving Prime Minister Theresa May the power to start the country's departure from the European Union.

The government is hoping to rush through the legislation in time to trigger Article 50 of the EU's Lisbon Treaty, which opens two years of divorce negotiations, by the end of March.

While the two-clause bill is expected to swiftly pass the lower House of Commons, it could be delayed in the upper House of Lords, where May's Conservative Party does not have a majority.

Its ultimate adoption is in little doubt, however.

"It is not a bill about whether or not the UK should leave the EU, or how it should do so," Brexit minister David Davis said as he opened the first, two-day debate on the legislation. "It is simply about implementing a decision already made, a point of no return already passed."

The government had originally sought to bypass parliament, insisting it had the power to trigger Article 50 on its own, but the Supreme Court last week ruled it must consult lawmakers.

A majority of both houses of parliament opposed Brexit, but May has urged them to respect last year's historic referendum, when 52 per cent of Britons voted to leave the EU.

At just 143 words, the "European Union Notification of Withdrawal Bill" has been tightly drafted, making it difficult to amend either to delay the government's plans or to tie its hands in the talks.

Opposition Labour leader Jeremy Corbyn has promised not to block the bill, although his party - like the wider country - is deeply divided and dozens of his MPs are expected to rebel.

"I am not failing to trust the people, I just disagree with some of them and I agree with the 48 per cent who chose to remain," Labour MP Paul Farrelly said during the debate on Tuesday.

But fellow Labour lawmaker Emma Reynolds urged MPs to back the Article 50 bill: "Those of us on the Remain side might not like the (referendum) result, but we have to accept it."

The Liberal Democrats' Sarah Olney, elected in December on an anti-Brexit platform, said parliament had been presented with no evidence the country would benefit from immediately triggering Article 50.

"We are effectively being asked to jump out of an aeroplane, without knowing whether or not we are securely attached to a parachute," she said.

Dozens of May's Conservative MPs are also opposed to her plan to pull Britain out of Europe's single market, but most of her party's lawmakers have promised to back the government as long as ministers keep parliament updated and involved in the process.

May has already promised MPs a vote on the final divorce deal.

Pro-Brexit Conservative MP Steve Baker warned fellow lawmakers they would "suffer the kind of political implosion in this country which we can scarcely imagine" if they refused to pass the bill.

After the preliminary debate, a vote on Wednesday allowing the bill to move to its next stage is viewed as largely procedural.

Over three days next week, however, the Commons will consider substantive amendments on issues such as access to Europe's single market.

There are also demands for greater involvement from the devolved parliaments of Wales, Scotland and Northern Ireland, with the latter two voting in June for Britain to remain in the EU.

The bill will then move to the Lords for debate from February 20, with the government hoping for their approval by 7 March.

The Times newspaper said this could mean that Britain - the first country to vote to leave the EU - would trigger Article 50 at an EU summit on 9-10 March, although Downing Street dismissed this.