By Guy Faulconbridge and Michael Holden

LONDON (Reuters) - Many British voters were fooled into voting to quit the European Union without realizing there was no credible plan for an exit, so lawmakers must decide how and whether to leave, said the investment manager behind a Brexit legal challenge.

Gina Miller, a co-founder of London fund manager SCM Private, is the main claimant in a growing queue of litigants hoping to force Prime Minister Theresa May to let parliament decide when, how and whether to leave the EU, rather than taking such decisions herself following last month's referendum.

In her first interview since taking her case to London's High Court, Miller said she wanted Britain to remain in the EU and the best decision would be to renegotiate membership and reform the EU.

"I believe that people have been fooled," Miller, 51, told Reuters in her office in London's exclusive Knightsbridge area. "They have not realized that the referendum was not legally binding and secondly there is no Brexit plan.

"That's what my action is about, we need to have a better-informed debate on what this momentous decision means."

The legal action is the first concrete sign since the June 23 referendum that implementation of the Brexit vote could be held up or at least amended.

But the action faces one big problem. It is attempting to get British lawmakers to do something most do not want to do: vote against the views of many of their own constituents.

"What MPs are desperate to do is to avoid voting on this at all costs," said Anand Menon, professor of European politics at King's College in London.

Turnout in the referendum was 72.2 percent. A total of 17.41 million people, or 51.9 percent, voted to leave while 16.14 million, or 48.1 percent, voted to remain.

The Brexit result, which historians say compares to the 1991 fall of the Soviet Union in its significance, unleashed political and financial turmoil and raised questions about the future of Britain and post-World War Two European integration.

May, who did not support leaving, says Brexit means Brexit and the vote will be respected.

But at least seven lawsuits have been brought to force the government to accept that parliament should decide whether Britain should trigger Article 50 of the Lisbon Treaty, the formal exit process, rather than the prime minister.


Miller, who describes herself as a "born fighter" and "a failed lawyer", felt sick when she heard the Brexit result.

"The whole day on Friday I felt physically sick because I thought: I don't think people know the ramifications of this, of what's happened, and I felt really sorry that people had been tricked and fooled.

"I'd like them to make a more informed decision," she said. "It's about giving people the tools to make the best decision for themselves, it's not about taking away their choice to make their decision."

London law firm Mishcon de Reya is acting for Miller though it is not charging her, she said. On Tuesday, the High Court ruled that Miller's case should be the lead action in the Article 50 claims.

Among others expected to join the action are a British hairdresser and a group of Britons living in France.

The argument for parliamentary approval before invoking Article 50 is based on a view of the authority of EU law in Britain.

Under this reasoning, EU law applies because of the European Communities Act of 1972, so the executive cannot undermine rights given by parliament by triggering Article 50.

Government lawyers dispute that and say the executive has the right to notify the EU of Britain's intention to leave. The government has rejected a petition signed by more than 4 million people demanding a second referendum.

A hearing is scheduled in mid-October and Miller's case might go straight to the Supreme Court to speed up the process.

If she is successful, parliament would have to debate Brexit, vote on Article 50 and then pass legislation.

Miller said hundreds of people - from many backgrounds and professions - were lending their support and their names would be published soon.

But some had faced fierce criticism, including "lot of very nasty emails and tweets targeted to them personally and their businesses".

She showed Reuters one email from someone who claimed they would have invested money with SCM, which has about 100 million pounds in its portfolios, but had been deterred because of her legal action.

"You are obviously a devious lady," the email read. Miller laughed it off said she deletes most such messages.

When asked about claims that the British establishment was trying to entangle, delay and ultimately prevent Brexit, Miller said: "If they did their homework, I'm possibly the most anti-establishment person they would ever come across."

Many Brexit voters were already regretting their decision, said Miller, who supports the opposition Labour Party.

She said significant numbers were saying they did not realize what they had voted for. "That's what I'd like to put right, which is a grown-up, considered, serious debate about the consequences of us leaving the EU."

Miller outlined three possible outcomes: Brexit, but arrived at in a considered manner followed by triggering Article 50; the middle ground of a so called "Brexit-Lite"; or another type of EU membership.

"I would prefer us to have a different sort of membership," she said.

(Writing by Guy Faulconbridge; editing by Giles Elgood)