British Prime Minister Theresa May has narrowly avoided a defeat in parliament at the hands of pro-EU MPs from her own party, fending off a rebellion that had threatened to deepen a crisis over her Brexit strategy.

Parliament voted 307 to 301 against an amendment to trade legislation in a vote that was seen as a test of Ms May’s ability to deliver a British exit from the European Union and keep her job.

The amendment would have required the government to try to negotiate a customs union arrangement with the EU if, by January 21, 2019, it had failed to negotiate a frictionless free trade deal with the bloc.

But there was no guarantee that the issue of retaining a form of customs union – which pro-EU MPs see as vital to preserving industrial and commercial supply chains – would not resurface.

Tuesday’s legislation was technical in nature -–focusing on converting trade deals between the EU and third countries into bilateral deals with Britain – and was not originally intended to define new trade policy.

However, the government did suffer an unexpected defeat on a separate amendment, which means it will now be required to seek an agreement that allows Britain to have continued participation in the European medicines regulatory framework.

“Very significant defeat of govt tonight on European medicines regulation amendment. Near miss on customs union amendment. Margin is closing on these votes & we will keep at it,” tweeted Keir Starmer, Brexit spokesman for the opposition Labour Party, which favours a customs tie with the EU.

The Department for International Trade said Britain was always seeking to be part of the medicines framework. It said it objected to the amendment because it requires the government to take “all necessary steps” to join while the government wants to join only if it can negotiate reasonable terms.

Ms May is expected to face many more challenges to her strategy after a summer break as she works her way through a mountain of Brexit-related legislation.

Any final deal with the EU will also require ratification by a bitterly split parliament.

Highlighting the fine margins Ms May is dealing with, Tuesday’s victory required the votes of four pro-Brexit Labour opposition MPs who backed the government in defiance of their party’s instructions.

Ms May will be back in the firing line on Wednesday when she takes questions from MPs in parliament before appearing in front of a parliamentary committee, and then leading a Conservative Party meeting.

The close shave was Ms May’s third this week as she presents legislation on one of the most important and divisive decisions in modern British history with only a minority government, and her Conservative Party at war with itself.

By prevailing in Tuesday’s vote, she avoided for now the prospect of having to go back on her word that Britain will not be part of any customs union after leaving the EU – something that would have enraged the pro-Brexit wing of her party.

-AAP