The United Kingdom High Court foiled Prime Minister Theresa May’s plans for Brexit when it ruled Thursday that the government cannot begin the process of the U.K. leaving the European Union without a vote from the parliament.

The ruling, delivered by the lord chief justice, Lord Thomas of Cwmgiedd, said the government cannot trigger Article 50 of the Lisbon Treaty, which initiates the two-year process of a member country leaving the E.U., without consulting the parliament. The E.U.’s 27 other members cannot negotiate the terms of Britain’s exit from the bloc till Article 50 is triggered.

May said earlier that her government hoped to start the process by early next year but the High Court’s decision will likely push back the U.K.’s exit from the bloc.

The lord chief justice reportedly said, “The most fundamental rule of the U.K. constitution is that parliament is sovereign.”

The U.K. government said it would appeal to the supreme court against this decision. The hearing is expected to occur Dec. 7 or Dec. 8.

“The country voted to leave the European Union in a referendum approved by act of Parliament,” a U.K. government spokesman reportedly said. “And the government is determined to respect the result of the referendum.”

The U.K. overwhelmingly voted to leave the E.U. in July this year with the “leave” campaign garnering about 51.9 percent of votes.

Nigel Farage, the outgoing leader of the U.K. Independence Party and one of Brexit’s biggest supporters, said he is worried about the country heading for a “half Brexit.”

“We are heading for a half Brexit ... I’m becoming increasingly worried. I see MPs from all parties saying, oh well, actually we should stay part of the single market, we should continue with our daily financial contributions,” he reportedly said. “I think we could be at the beginning, with this ruling, of a process where there is deliberate, wilful attempt by our political class to betray 17.4m voters.”

He vowed, however, to once again take up campaigning for the U.K. to leave the E.U. if the country doesn’t leave the bloc by 2017. But, May’s office reportedly said it had every intention of fulfilling the promise of initiating the Brexit process by the end of March in 2017.