In or Out? Britain faces vital EU membership vote June 23

The prospect of a British exit — a Brexit — hangs over the future of the EU, which is already reeling from a prolonged refugee crisis, a series of lethal attacks, and a financial meltdown that has threatened the future of the euro single currency that is used by 19 member states, though not Britain.

The campaign will move to television in the coming weeks, with combative town hall meetings and debates that will include Cameron and right-wing U.K. Independence Party leader Nigel Farage of the "Leave" side.

Some experts believe a decision to leave could jeopardize the future of the EU itself by removing a key member, and also set in motion the possible disintegration of the United Kingdom, because Scottish voters loyal to the EU might choose to break away from Britain via a new referendum.

The "stronger in Europe" campaign argues that the benefits of membership — including easy trade ties, the free flow of goods and people, and the ability to live and work in other European countries — outweigh the cost of dealing with the sometimes-stifling Brussels bureaucracy.

A severing of EU ties would shelve a whole series of trade arrangements that make it easy to do business across European borders, impacting the global banks, trading houses and insurance companies that have made London a financial powerhouse.

A decision to leave would also have a profound effect on the lives of younger Europeans, who have flocked to London and other British cities where jobs are relatively plentiful compared to the capitals of Spain, Italy, Greece and the EU's eastern European members.