U.K. Prime Minister Theresa May, fighting a High Court judgment that she needs Parliament’s approval to formally begin Brexit talks, faces a new legal challenge to prove that Britain’s vote to quit the European Union gives her a mandate to exit the EU’s single market. Peter Wilding, a “Remain” campaigner who founded the research institute British Influence, and Adrian Yalland, a former Conservative Party parliamentary candidate who supports Brexit, are seeking a judicial review of the government position on the single market, according to a statement e-mailed on Sunday. The men, both lawyers, want courts to rule on whether the government must seek a separate mandate from Parliament to pull out of the European Economic Area, which grants access to the single market. “For Brexit to be legitimate it must be both democratically mandated and lawfully applied, domestically and internationally,” Yalland said. “I fully understand why the referendum campaign created an expectation in some minds that leaving the EU meant leaving the EEA, but that expectation is neither a democratic mandate for leaving the EEA nor a legal mechanism to do so.” The case will further complicate May’s effort to begin formal Brexit talks by the end of March by triggering Article 50 of the EU’s Lisbon treaty. She’s seeking to bypass Parliament, and government lawyers spent four days at the Supreme Court last week appealing a High Court decision that she must seek lawmaker approval. The Supreme Court will rule next month. “The U.K. is party to the EEA Agreement only in its capacity as an EU member state,” the government’s Department for Exiting the European Union said in an e-mailed statement. “Once the U.K. leaves the EU, the EEA Agreement will automatically cease to apply to the U.K.” Yalland and Wilding argue that because Britain signed up to the EEA in a separate treaty that was approved by Parliament, lawmakers must separately agree to triggering Article 127 of that agreement in order to license a single market pullout. Their legal action was reported earlier by the Sunday Times.