Britain's parliament has dealt Prime Minister Theresa May's government a narrow defeat by passing an amendment designed to put roadblocks in place to make a "no-deal" Brexit more difficult.

Legislators on Tuesday backed an amendment to the Finance Bill that would prohibit the government from spending on preparations to leave the European Union without a deal unless authorised by parliament.

The 303 to 296 defeat highlights May's weak position as leader of a minority government, a divided party, and a critical parliament just days before she is due to hold a pivotal vote on whether to approve the Brexit deal she has negotiated with the EU.

Leader of the main opposition Labour Party Jeremy Corbyn described the result as an "important step" towards preventing a no-deal Brexit.

"It shows that there is no majority in Parliament, the Cabinet or the country for crashing out of the EU without an agreement," Corbyn said in a Tweet.

This vote is an important step to prevent a no deal Brexit.



It shows that there is no majority in Parliament, the Cabinet or the country for crashing out of the EU without an agreement. https://t.co/frbXnPhhM8 — Jeremy Corbyn (@jeremycorbyn) January 8, 2019

"Theresa May must now rule out no deal once and for all," he added.

Looming showdown

The UK is poised to leave the EU on March 29, two years after it triggered Article 50, the exit clause in the EU's constitution, and kick-started arduous negotiations with European leaders over a divorce deal.

With less than three months until Britain leaves the EU, May is struggling to win approval for her Brexit deal, which was settled on in November after more than a year of back-and-forth negotiations between London and Brussels.

Legislators are set to vote on the withdrawal deal on January 15.

But parliamentary opposition to her deal remains fierce, heightening fears Britain could crash out of the EU without a deal.

A no-deal exit is the default scenario if May's deal is rejected, and the prospect of possible supply chain disruption, medicine shortage and blocked ports has in recent weeks pushed companies and the government to ramp up contingency planning.

Al Jazeera's Jonah Hull, reporting from the capital, London, said Tuesday's defeat could mark the "beginning of constitutional warfare in parliament" ahead of next week's vote.

"This is likely to be the first salvo in a pretty sustained campaign among the opponents of a no-deal outcome to try and frustrate that outcome by putting amendments like this on every piece of legislation they can, essentially searching for every spanner they can find to throw into the works and frustrate the machinery of government, making no deal unworkable," Hull said.

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The main sticking point in May's deal is the safety net "backstop" measure - which would keep open the border between EU member Ireland and Northern Ireland, which is part of the UK, following Brexit by temporarily aligning Britain with EU trade rules.

Critics of the backstop argue it could tie the UK into the EU's orbit indefinitely.

In an effort to assuage MPs concerns, May has lobbied her European counterparts and officials in Brussels to make concessions on the clause.

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The EU, for its part, has insisted that the withdrawal agreement cannot be renegotiated but has made clear the backstop is meant only as a temporary measure of last resort.

Some legislators have said they will vote against the deal because they prefer another referendum on Britain's EU membership.

On Sunday, May warned critics of her departure plan risk damaging Britain's democracy and weakening its economy by opposing her deal.

She also alleged her deal was the only one that respects the UK's 2016 referendum result, in which 52 percent of Britons voted to leave the EU.

The UK's central bank has warned that Britain's gross domestic product could shrink by up to eight percent in such a scenario. The government, for its part, has forecast a potential economic slump of more than nine percent in the wake of a no-deal Brexit.