British MPs were due to spend several more hours on Wednesday debating a two-clause bill on the UK's decision to leave the European Union, ahead of a vote later in the evening.

The short-form legislation is the legal mechanism to trigger Brexit negotiations. British Prime Minister Theresa May has confirmed she will publish a full policy document detailing her negotiation plan on Thursday.

The first day's parliamentary discussion continued until midnight on Tuesday over the merits of the short text, which gives May the power to start the country's departure from the 28-member bloc.

The debate began with Brexit minister David Davis telling MPs that the law was "not a bill about whether or not the UK should leave the EU, or how it should do so."

"It is simply about implementing a decision already made, a point of no return already passed," he said, referring to the June 23 referendum when British voters decided by 52 to 48 percent to leave the bloc.

MP warns of post-Brexit fantasy

But Davis quickly faced dissent from within his own Conservative party, when former Finance Minister and Home Secretary Kenneth Clarke said he would vote with his conscience against the bill.

Describing the government's Brexit plan as "Alice in Wonderland," Clarke mocked May's recent visits to the US and Turkey, questioning whether both countries would be willing to further open their markets to Britain if it loses access to the EU's single market.

"Apparently you follow the rabbit down the hole and emerge in a wonderland where suddenly countries throughout the world are queuing up to give us trading advantages and access to their markets that previously we've never been able to achieve.

"Nice men like President Trump and President Erdogan are just impatient to abandon their normal protectionism and give us access!" Clarke said to laughter and applause from fellow MPs.

Tough decision

Several MPs of most parties said they would support the bill despite having strong doubts about Britain's future success outside the EU. Many of them are personally at odds with the constituencies they represent, where voters decided overwhelmingly for or against leaving the bloc.

But most politicians have promised to back the government as long as ministers keep parliament updated and involved in the process.

The main opposition Labour party has promised not to block the Brexit vote, but some of its MPs say they will defy party orders and vote against the bill.

Little wiggle room

Wednesday's parliamentary vote, which will take place at around 1900 UTC, follows a Supreme Court ruling ordering the British government to consult MPs before it triggers Article 50 of the EU Lisbon Treaty, the formal proceedings to leave the bloc.

In response, the British government has devised the 143-word "European Union Notification of Withdrawal Bill," which leaves MPs little room to amend or delay the text.

Wednesday's vote is expected to pass the lower House of Commons easily, but analysts said it could face hostility in the upper House of Lords, where May's ruling Conservative Party does not have a majority. It will be debated in the Lords from February 20, with approval expected by March 7.

May has promised to trigger Article 50 by the end of March and has said that parliament will get to vote on the final deal on Britain's new relationship with the EU.

mm/kms (AFP, AP, dpa, Reuters)