There is a conspiracy theory that a Brexit won't actually happen if the public votes for it

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The House of Commons.

A really crucial detail about the upcoming EU referendum has gone virtually unmentioned and it is probably the most crucial detail: Parliament doesn't actually have to bring Britain out of the EU if the public votes for it.

That is because the result of June 23 referendum on Britain's EU membership is not legally binding. Instead, it is merely advisory, and, in theory, could be totally ignored by UK government.

This incredible detail is explained in a new blog post by Financial Times columnist and legal expert David Allen Green.

Green says that no legal provision was included in the EU referendum legislation that requires UK Parliament to act in accordance with the outcome of the EU referendum.

Instead, what will happen next if the public votes for a Brexit will be purely a matter of parliamentary politics.

The government could decide to put the matter to parliament and then hope to win the vote, Green says. Alternatively, ministers could attempt to negotiate an updated EU membership deal and put it to another referendum. Finally, the government could just choose to totally ignore the will of the public.

Business Insider's live chart below shows how public opinion is shifting towards voting for a Brexit.

Business Insider

Pro-EU MPs could even argue that ignoring the public's will would be parliamentary sovereignty in practice - something that Leave campaigners argue has been conceded to Brussels.

The only way that a Brexit vote would have weight in law would be if the government decided to invoke Article 50 of the Lisbon Treaty. This is when an EU member state chooses to activate the process of withdrawing from the 28-nation bloc.

Article 50 would make Britain's EU membership a legal matter. However, even if the June 23 referendum produces a Leave majority, the government would not be obliged to invoke the legislation.

As Green says (emphasis ours):

A vote for Brexit will not be determinative of whether the UK will leave the EU. That potential outcome comes down to the political decisions which then follow before the Article 50 notification. The policy of the government (if not of all of its ministers) is to remain in the EU. The UK government may thereby seek to put off the Article 50 notification, regardless of political pressure and conventional wisdom.

This has to go down as one of the largest pieces of small print in British political history.

The overwhelming majority of the British public is probably totally unaware of this legislative loophole. As far as most Brits understand, Britain will no longer be an EU member if Leave wins next week's referendum.

Interestingly, parliament choosing to ignore the British public isn't as unthinkable as conventional wisdom leads us to believe. In fact, according to the BBC, MPs have already discussed the possibility.

Speaking to the BBC earlier this month, an unnamed pro-EU MP said: "We would accept the mandate of the people to leave the EU. But everything after that is negotiable and parliament would have its say. The terms on which we leave are entirely within my remit as a parliamentarian and that is something for me to take a view on."