What if? The uncharted waters of 'Brexit'

A British exit from the European Union would leave the country in uncharted territory, no country having ever travelled that road, which spells freedom to eurosceptics but doom to the pro-Europe camp.

As Britain gears up for a membership referendum, European Commission President Jean-Claude Juncker insists there is no plan B, while Downing Street maintains it has made no contingencies.

After securing a deal on Britain's 'special status' in the EU at a summit on Friday, Prime Minister David Cameron said he would campaign for his country to stay in and warned those clamouring for a divorce that a post-EU future might be far from rosy.

"We should be suspicious of those who claim that leaving Europe is some automatic fast track to some land of milk and honey," Cameron said, adding that "Brexit" would be "a leap into the unknown".

A "War Game" held in London last month attempted to simulate post-Brexit negotiations on Britain's place in Europe, but even there discussions quickly turned toxic.

As a first step, Britain and the EU must "negotiate and conclude an agreement... setting out the arrangements for its withdrawal", as required by Article 50 of the Lisbon Treaty.

If no agreement is reached after two years, Britain would automatically be ejected from the union, unless both parties agreed to an extension.

At the heart of discussions will be whether Britain remains in the European Economic Area (EEA), like other non-EU members Norway and Iceland, or whether it quits the single market altogether.

The City of London, Europe's most important financial centre, is hostile to a Brexit and its big hitters have already planned for various post-EU scenarios.

HSBC, Europe's biggest bank, has warned that 1,000 jobs could shift from London to Paris.

- Immigration clampdown -

According to a study by the think-tank Open Europe, Britain's GDP would be 2.2 points lower in 2030 if Britain leaves the EU, in its worst-case scenario, with a loss of 0.8 percent deemed most likely.

Despite Brexit's many unknowns, the government would likely clamp down on immigrants, starting with migrants from Eastern Europe, whom eurosceptics believe are being lured by Britain's welfare system.

Parliament would be able to strike down EU laws written into British law, which currently state that the welfare system must treat workers from other parts of Europe as it does British citizens.

France would have to decide whether to continue to host British border police, or whether it would lift the controls to allow migrants currently stranded in Calais to travel on to England through the Eurotunnel.

Meanwhile, EU citizens already resident in Britain could suddenly find themselves treated as any other foreigner, requiring residence and work permits.

Remaining EU members could respond in kind, leaving Britons on the continent in need of visas and permits.

- Imperial victory -

In the key areas of security and defence, it seems likely that all parties would conclude the need to continue cooperating closely on defence and counter-terrorism.

But a Brexit could leave Britain weakened on the world stage as it loses its role as a gateway to Europe for the United States and China.

Both US President Barack Obama and his Chinese counterpart Xi Jinping have publicly called for Britain to stay in the EU.

Britain could be further weakened by the prospect of losing Scotland, whose devolved government has signalled it would demand another independence referendum in the event of a Brexit.

The ruling Scottish National Party (SNP) claims that it is being forced out of the EU against its wishes, and believes it would have a stronger chance of winning independence than it did in 2014, when Scotland voted by 55 percent to 45 percent to remain in the United Kingdom.

The consequences would be dire for Prime Minster David Cameron, who would go down in history as the man who empowered the country's eurosceptics to drive Britain out of the EU.

Political commentators are already fantasising about charismatic London mayor Boris Johnson, a die-hard eurosceptic, replacing Cameron should the vote go against the current prime minister.

Another issue that carries weight, literally, among Britain's eurosceptics will also be settled.

The 2009 European regulation imposing the use of the "metric" system will finally be able to be binned, signalling a return to "imperial" measurements and victory for "metric martyr" campaigners.