U.K. Prime Minister Theresa May faces a battle with pro-European lawmakers in her Conservative Party after they grudgingly backed her plan to trigger Brexit negotiations by the end of March in its first test in Parliament.

The House of Commons on Wednesday approved by 498 votes to 114 to allow May to start divorce talks with the European Union. But with more parliamentary hurdles ahead, lawmakers warned their backing shouldn’t be mistaken for unconditional support to negotiate Brexit freely.

The government will publish a written outline of its Brexit plans on Thursday and it will lay out details of the clean break with the EU that the premier wants, including leaving the EU single market, overhauling membership of the customs union, and controlling migration, with a period of transitional arrangements to help businesses adjust.

“The battle’s only just started,” said former Chancellor of the Exchequer Kenneth Clarke, the one Conservative who said he’d oppose the bill. “We’ve been in a very unreal, silly world since the rather startling result of the referendum. We’re on a voyage of discovery now with a sketchy outline of the negotiating position.”

Under May’s plan, Parliament will have to decide on a series of issues that will “completely divide” political friends and the wider country, former Chancellor of the Exchequer George Osborne told lawmakers before the vote in London. Her government will have to grapple with controversies over setting migration limits, state support for farms and businesses and free trade policies, he said.

“I will be in those fights in the couple of years ahead,” Osborne, who campaigned to stay in the EU, told lawmakers.

May didn’t want to give Parliament a say on triggering Article 50 of the EU’s Lisbon treaty, formally launching Britain on the path out of the EU. But she lost a court case last week and was forced to seek approval from lawmakers.

As the main opposition Labour Party is backing May’s law, it is almost guaranteed to pass its final stages sometime in March. Still, the Commons debate leading up to the vote contained plenty of clues that the premier faces trouble ahead.

“The government has chosen, and I respect this decision, not to make the economy the priority in this negotiation,” Osborne said, pointing to May’s “red line” of controlling immigration. “The European Union is not prioritizing the economy either in these negotiations. Both sides at the moment are heading for a clean break.”

Osborne and other senior Conservatives expressed reservations about May’s trajectory toward a hard Brexit, an arrangement that would see complete control of immigration, laws and budget even if it means giving up membership of the single market and customs union.

They will seek to use the so-called white paper as a reason to re-write May’s bill when it enters the next stage of detailed parliamentary scrutiny next week.

The bill will eventually get to the unelected upper house. The government has no majority in the 805-member Lords, in which the anti-Brexit Liberal Democrats have more than 100 lawmakers.

Former Labour minister Peter Hain, now a member of the House of Lords, tweeted that “I and others will vote against bill” when it goes to the upper chamber later this month “to block May’s right-wing Brexit nightmare for Britain.”

Bloomberg News