The U.K. could seek to model its new relationship with the European Union on trade rules used by Switzerland or Norway, Brexit Secretary David Davis said as lawmakers debated plans for leaving the bloc. The government is considering about four different options for the country’s future relationship with the customs union, which imposes common tariffs on external countries while allowing EU members tariff-free trade with each other, Davis told Parliament in London on Wednesday. “There are several options of customs union; one is shown by Norway, which is in the single market but not in the customs union,” Davis said. “You’ve got one shown by Switzerland, which is neither in the customs union nor in the single market but has a customs agreement, so there are a whole series of options that exist.” Members of the House of Commons were debating a motion that backs Prime Minister Theresa May’s schedule for beginning the formal Brexit process, requiring the U.K. to give notice of its departure from the bloc under Article 50 of the EU’s Lisbon Treaty by the end of March. May has insisted she is not seeking to replicate another country’s relationship with the EU but instead wants a patriotic “red, white and blue Brexit” that works for the U.K., referring to the colors of the British flag. The premier says she wants the U.K. to strike new trade deals with other countries outside the EU after Brexit, leading the world in a new free-trade era, but officials say this cannot happen until the U.K. leaves the European customs union. Davis was challenged by members of his own ruling Conservative Party, as well as opposition lawmakers, to set out whether the U.K. will remain inside or outside the customs union. The U.K. doesn’t face a “binary” choice, the Brexit secretary said. “There are about four different possibilities and we are still assessing that,” he added. The complexity of reaching a consensus in Parliament was underlined by rank-and-file lawmakers in May’s Conservative Party. Former Work and Pensions Secretary Iain Duncan Smith, a prominent campaigner for Brexit, said it would be preferable to remain in the EU than to leave the bloc while staying in the customs union. Former Chancellor of the Exchequer Kenneth Clarke said the June referendum wasn’t a vote on the issue. Mystery to Most “Our discussion on whether we should be in the customs union, and the consequences one way or another were not decided by the referendum,” Clarke said. “These choices, which the ministers are now struggling with, for which they should be accountable to us, would have been a mystery to 99 percent of the people listening to the debate and voting in the referendum.” Davis’s comments came as he challenged the government’s opponents to back its plan to trigger the start of Brexit by March next year during the debate, which gives lawmakers their first chance to vote on the timetable for leaving the EU. May has promised to set out her plan for Brexit and allow lawmakers to scrutinize it before she triggers Article 50, but some members of the opposition have already said they will vote against the non-binding motion on Wednesday. Britons voted by 52 percent to 48 percent in June to quit the bloc. “The vote tonight will be the first opportunity for members of this house to decide whether or not they support the government’s timetable of triggering Article 50 by the end of March 2017,” David Lidington, a senior minister in May’s cabinet, told Parliament before the full debate began. Any lawmaker who votes against the motion “will in my view be seeking to thwart the outcome of the referendum in the most profoundly undemocratic fashion.” ‘Vitriolic Abuse’ Former Attorney-General Dominic Grieve, a Tory, called for an end to the “vitriolic abuse” and death threats aimed at supporters of Remain in the referendum. Opening the main debate, the opposition Labour Party’s Brexit spokesman, Keir Starmer, said he does not want to “frustrate” the process “or to delay the timetable,” but he called on the government to produce a minimum plan for what it wants to achieve in talks with the EU. “We’re not going to have a situation where the government seeks a vote in a vacuum or produces a late, vague plan,” Starmer told lawmakers. May’s team must set out a strategy that includes “enough detail and clarity to end the circus of uncertainty” on the U.K.’s membership of the EU single market and customs union, Starmer said. ‘Overwhelming Evidence’ “There is no mandate for hard Brexit; there is no consensus for hard Brexit,” Starmer said. The “overwhelming evidence” is that the public “do not want hard Brexit.” Former Business Minister Anna Soubry, a Conservative, said that even though the government had agreed to hold a series of debates on Brexit, the discussions so far haven’t been “good enough.” “The debates we now need to be having are about the value of the single market,” Soubry said. “Let’s thrash it out. Let’s hear why some say we shouldn’t be in it. Let’s talk about the customs union. Let’s talk about the peril of tariffs. Let’s talk about immigration—the positive benefits of immigration and some of the downsides—but let’s have these debates and most importantly, let us take part in this: Parliament.” Separately from the outcome of Wednesday’s debate, the government is also appealing against a High Court decision that it needs the approval of Parliament before triggering Article 50. Four days of Supreme Court hearings began on Monday, with the court expected to rule next month. A defeat in the court would create much more difficulty for the government, necessitating the introduction of a bill that could delay the Brexit process. Davis said it’s “inconceivable” that lawmakers in the U.K. Parliament will not get a vote on whether to accept the final Brexit deal, once negotiations with the EU have finished. But the minister warned that whatever the government plans, the U.K. might not get what it wants from the negotiations in Brussels. “This is a negotiation; it’s not a policy statement, and therefore where we’re aiming for may not be the exact place we end up at,” Davis said. The talks will be “the most important and complex negotiations in modern times,” he said.