LONDON (AP) — British Prime Minister Theresa May faced more rebellion in Parliament on Tuesday over her plans for the country’s exit from the European Union, with lawmakers voting on a Brexit trade bill a day after she narrowly avoided a parliamentary defeat.

Pro-EU lawmakers — from both May’s Conservatives and the opposition Labour Party — want Britain to join a European customs union if there is no trade agreement with the bloc by January, two months before Britain is due to leave.

They are seeking to change the wording of the Trade Bill, which gives the government the power to set up new international trade relationships after Britain leaves the EU in March.

May’s government on Monday avoided a humiliating defeat in Parliament when it narrowly won another vote over her Brexit customs bill — but only after reluctantly accepting amendments put forward by Brexit hardliners.

The government won by just three votes, underlining the fragility of May’s support as she tries to find a way to move the complex Brexit process forward.

May’s challenges came as some British politicians on Tuesday again questioned the legitimacy of the Brexit vote after the electoral watchdog said the official group campaigning for Britain’s exit in the 2016 referendum broke electoral laws.

The Electoral Commission said the “Vote Leave” group, backed by senior politicians including former foreign secretary Boris Johnson, failed to declare 675,000 pounds ($894,000) it spent with Canadian data firm Aggregate IQ. The undeclared spending meant the campaign group exceeded the 7 million-pound legal spending limit by almost 500,000 pounds.

The commission said it found significant evidence that Vote Leave did this by funneling cash to a small, unregistered pro-Brexit youth group, BeLeave.

Those who wanted Britain to leave the EU narrowly won the 2016 referendum, with 52 percent of voters supporting Brexit.

“This news makes the narrow referendum result looks dodgier than ever. Its validity is now in question,” said Labour lawmaker David Lammy.

The election fraud probe became entangled with inquiries into Facebook’s use of private data, because Aggregate IQ had links with Cambridge Analytica, the British firm accused of using data from tens of millions of Facebook accounts to help Donald Trump win the 2016 U.S. election.

Whistleblowers had alleged that Brexit campaigners paid Aggregate IQ to send targeted ads and that their actions may have unfairly influenced the referendum’s outcome.

Vote Leave was fined 61,000 pounds ($80,100). BeLeave founder Darren Grimes, a student at the time, was fined 20,000 pounds ($26,300). The commission said Grimes and an official from Vote Leave have both been referred to police “in relation to false declarations of campaign spending.”

Grimes and Vote Leave both deny wrongdoing.

Another pro-Brexit organization, Leave.EU, was fined 70,000 pounds ($92,000) in the spring for overspending and filing inaccurate records.