British Prime Minister Theresa May’s government has suffered a crucial defeat in parliament that throws up a potential new obstacle to a no-deal Brexit.

MPs opposed to leaving the European Union without an accord narrowly won a vote that will force the UK government to secure explicit parliamentary approval for a no-deal Brexit before it can use certain powers relating to taxation law.

The motion was passed by just 303 votes to 296.

The defeat highlights Ms 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.

“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,” opposition leader Jeremy Corbyn of the Labour Party said Wednesday (Australian time).

Speaking before the vote, Treasury minister Robert Jenrick said the proposal’s only impact would be to make the country “somewhat less prepared” for a no-deal outcome.

With less than three months until Britain leaves the EU, Ms May is struggling to win approval for her Brexit deal.

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

Some Brexit supporters say a no-deal exit is the only way to truly leave the bloc, and that warnings of the economic consequences have been overblown to drum up support for Ms May’s deal.

But, a majority of MPs from across the political spectrum opposed to a no-deal exit have now established their political significance and promised to keep making it harder for the government to leave without a deal.

Their amendment to legislation implementing last year’s budget states that powers to amend tax laws to make them work after Brexit could only be used if a deal was agreed, Brexit was cancelled, or, if the government had parliamentary approval to proceed with a no-deal exit.

Effectively this constrains the government’s ability to act to keep its taxation system working smoothly if it pursues a no-deal Brexit against the wishes of a majority in parliament.

It is not an absolute block to leaving the EU without a deal because it does not negate the legislation already in place stating that Britain will leave the EU on March 29.

But it is a politically important victory for those opposed to no-deal, demonstrating that they have the ability to defeat the government.

One member of Ms May’s party, who co-signed the amendment, said he hoped similar tactics would be used on other pieces of legislation – a sentiment endorsed by others in the debate.

-with AAP