LONDON (Reuters) - As British lawmakers gather to debate whether to trigger the formal exit from the European Union, London lawyer Jolyon Maugham is building a case which he hopes will ensure that British voters can still reverse it.

The 45-year-old tax specialist, an opponent of leaving the EU, said it was right for Prime Minister Theresa May to start the exit process, but voters should also have a right to change their minds if parliament rejects the terms she negotiates.

He filed a case in Ireland last week that he hopes will go to the European Court of Justice (ECJ) in Luxembourg, to establish that Britain can reverse the exit process without requiring permission from the other 27 EU members.

Maugham told Reuters at his offices in central London he believed last year’s referendum meant parliament should vote to give May the authority to invoke Article 50 of the EU’s Lisbon treaty, which would pull Britain out of the bloc in two years.

“But we have a very uncertain political climate, economic climate, defence climate. The world is changing fast, we cannot know what the world will look like in two-years time,” he said.

“We cannot know that what we regarded as being in our national interest in June of last year will continue to be in our national interest in two years from now.”

By establishing the legal principle that voters can change their minds, Maugham said he hopes to keep the debate over the wisdom of Brexit alive as a political issue.

“If revocability is established, people will continue to ask the question whether in this fast changing and uncertain world Brexit is in the national interest,” he said.

A bill authorising May to invoke article 50 starts its route through parliament on Tuesday, expected to pass overwhelmingly.

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Although most members of parliament campaigned to stay in the EU, most, including the leader of the centre-left opposition Labour Party, say they will vote to uphold the result of the referendum, which “leave” won with 52 percent of the vote.

May has said that she will put the final deal she reaches at the end of negotiations to parliament for a vote, but that if parliament rejects it, Britain would have no choice but to leave the EU anyway, without any trade agreement with the bloc.

Maugham said that all-or-nothing stance means parliament cannot hold the government to account.

“It dawned on me without a viable alternative to rejecting the deal the government negotiated, parliament had no ability to control the government,” he said.

The legal question of whether Britain could change its mind and revoke article 50 is still open. Government lawyers have said the process cannot be reversed, but even David Davis, the cabinet minister in charge of leaving, said he was unsure.

“We don’t intend to revoke it. It may not be revocable. We don’t know,” he told parliament’s Brexit committee in December.

European Council President Donald Tusk has said he believed Britain could unilaterally withdrew its request to leave. One of Article 50’s authors, John Kerr, a former British ambassador to the EU, has also said Britain could change its mind.

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POSITIVE EUROPEAN VIEW

Maugham said his warm feelings for Europe were shaped by a year studying in Belgium on an EU-sponsored programme.

“It gives you a very fixed, positive view on the desirability of an open and progressive Europe,” he said.

He advised former Labour leader Ed Miliband on tax issues ahead of an election in 2015, and has set up the Good Law Project which aims to use legal challenges to “advance a progressive, responsive and forward-looking society”.

The day after the June referendum, as he attended a festival at his daughters’ school, he began formulating ways to hold a second vote. Within a week he was working on a crowdfunding project for what would become a successful legal challenge that forced May to seek parliament’s approval to trigger Article 50.

He has since raised 70,000 pounds through crowdfunding for his Dublin challenge.

One obstacle is that the ECJ does not rule on hypothetical questions. However Maugham said he did not view the question of whether Britain could reverse Article 50 as hypothetical.

Brexit supporters have accused those like Maugham of usurping democracy by using the law to thwart the will of the people. Gina Miller, the investment manager who fronted the successful court challenge requiring lawmakers to approve the triggering of Article 50, became a hate figure in Britain’s right-wing press and received death threats.

Maugham said there was nothing anti-democratic about establishing the principle that voters can change their minds.

“I do think the people were misled. What happens if the promises that Brexiteers made about sunlit uplands, extra money for the NHS (state-funded health service), no VAT (value-added sales tax) on domestic fuel, wonderful trade deals with the rest of the world -- what happens if those come to nought?”

“What if the economy turns sour? If the people come to agree with me, I think it would profoundly damaging to democracy and profoundly bad for a country we all love if we were forced to Brexit anyway.”