Cameron or his successor needs to decide when to invoke this - that will then set in motion the formal legal process of withdrawing from the EU, and give the UK two years to negotiate its withdrawal.
The article has only been in force since late 2009 and it hasn't been tested yet, so no-one really knows how the Brexit process will work, according to BBC legal correspondent Clive Coleman.
Mr Cameron, who has said he would be stepping down as PM by October, said he will go to the European Council next week to "explain the decision the British people have taken".
EU law still stands in the UK until it ceases being a member - and that process could take some time.
The UK will continue to abide by EU treaties and laws, but not take part in any decision-making, as it negotiates a withdrawal agreement and the terms of its relationship with the now 27 nation bloc…
Britain was a member of a free trade area in Europe before it joined what was then known as the common market. In a free trade area countries can trade with each other without paying tariffs - but it is not a single market because the member states do not have to merge their economies together.
The European Union single market, which was completed in 1992, allows the free movement of goods, services, money and people within the European Union, as if it was a single country.
It is possible to set up a business or take a job anywhere within it. The idea was to boost trade, create jobs and lower prices. But it requires common law-making to ensure products are made to the same technical standards and imposes other rules to ensure a "level playing field".
Critics say it generates too many petty regulations and robs members of control over their own affairs. Mass migration from poorer to richer countries has also raised questions about the free movement rule. Read more: A free trade area v EU single market
If economic performance deteriorates, the Bank of England could decide on a further program of quantitative easing, as an alternative to cutting interest rates, which would lower bond yields and with them annuity rates. So anyone taking out a pension annuity could get less income for their money.
The Bank of England may consider raising interest rates to combat extra pressure on inflation. That would make mortgages and loans more expensive to repay but would be good news for savers.
The Treasury previously forecast a rise of between 0.7% and 1.1% in mortgage borrowing costs, with the prime minister claiming the average cost of a mortgage could increase by up to £1,000 a year.
The Treasury argued during the referendum campaign that UK shares would become less attractive to foreign investors in the event of Brexit and would therefore decline in value, but in the longer term shares typically rise with company profits. Big exporters might benefit from the weaker pound, so the value of their shares might well rise, while importers might see profits squeezed.
About half of Conservative MPs, including five cabinet ministers, several Labour MPs and the DUP were also in favour of leaving.
One of the main principles of EU membership is "free movement", which means you don't need to get a visa to go and live in another EU country. The Leave campaign also objected to the idea of "ever closer union" and what they see as moves towards the creation of a "United States of Europe".
They also said Britain's status in the world would be damaged by leaving and that we are more secure as part of the 28 nation club, rather than going it alone…
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