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

Parliament voted 307 to 301 against an amendment to trade legislation that would have required the government to try to negotiate a customs union arrangement with the EU if, by Jan. 21, 2019, it had failed to negotiate a deal with the bloc that offered frictionless free trade for goods.

The narrow victory is May's third this week, underlining the difficulty she faces in passing legislation on one of the most divisive and important decisions in modern British history with only a minority government and a party at war with itself.

By winning Tuesday's vote, May has avoided 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 infuriated the pro-Brexit wing of her party.

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

The Trade Bill is focused on converting trade deals between the EU and third countries into bilateral deals with Britain. It is a technical bill and was not originally intended to define new trade policy.

Meanwhile, Britain's official Brexit campaign, Vote Leave, has been fined and reported to the police for breaking spending rules in the 2016 EU membership referendum, boosting calls for a second vote.

The Electoral Commission regulator said the winning side in the referendum had worked together with a smaller pro-Brexit group called BeLeave to get around rules limiting its campaign spending.

Vote Leave was fined £61,000 and BeLeave founder Darren Grimes, a fashion student, was fined £20,000 -- the maximum levy for an individual.

The report prompted angry questions in parliament, calls for the referendum to be annulled or re-run and claims that the entire Brexit process was now dubious.