The British House of Commons. Reuters TV / Reuters

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 the 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 the UK government.

(For more on this, see Business Insider's intriguing interview with the University of Westminster's Dr Peter Catterall: "Why a Brexit is unlikely to happen even if the public votes for it.")

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

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

This is unlike the last referendum held across Britain, the Alternative Vote referendum in 2011, where the outcome had a legal trigger and had to be acted on by the government of the time.

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. In the scenario of Britain's EU membership being put to a Westminster vote, barring no dramatic change in allegiances, it is likely that MPs would vote to keep the country in the 28-nation bloc.

This is because the vast majority of the 650 MPs identify as Euro-philes and would likely support a motion position to protect Britain's place in the EU.

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

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

The only way that a Brexit vote would have weight in law would be if the government decided to invoke Article 50 of the Treaty of Lisbon. 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. But even if the June 23 referendum produces a Leave majority, the government would not be obliged to invoke the legislation.

As Green says (emphasis added):

"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."

In an event attended by Business Insider on Wednesday night at the University of Westminster, former Deputy Prime Minister Nick Clegg said that he would not support a motion to ignore the will of the public, in spite of his strong pro-EU convictions.