Theresa May’s Brexit plans were thrown into turmoil yesterday after she suffered a High Court defeat that could trigger months of parliamentary warfare at Westminster over the terms of Britain’s departure from the EU.

Mrs May will today tell Jean-Claude Juncker, European Commission president, she is determined to face down a revolt by pro-EU MPs and stick to her plan to begin the Brexit process by the end of March 2017. “We have no intention of letting this derail our timetable,” her spokeswoman said.

The prime minister’s first line of defence is to go to the Supreme Court in early December to try to overturn a High Court ruling that she must grant a parliamentary vote before invoking Article 50, the EU treaty clause that triggers a two-year divorce process.