If the Scots vote to leave Britain in September, that could set off a chain reaction that leads to the rest of Britain’s quitting the European Union. This is a threat British pro-Europeans need to take seriously, given that a vote in favor of Scottish independence is quite possible, though the chances are still less than 50 percent.

Were it not for the Scotland factor, the risk of Britain’s exit from the European Union would be receding. A string of business leaders have in recent months argued that the economy would be damaged if Britain lost full access to the European Union’s single market.

All of the opinion polls by YouGov since March have shown a lead, varying between two and six percentage points, for Britain’s staying in the European Union. In the previous year, YouGov’s polls were consistently in favor of pulling out, with one showing a lead of 17 points for the “out” campaign.

This shift has happened even though the United Kingdom Independence Party, which wants to pull out of the European Union, is expected to get the most votes in this month’s European Parliament elections. The party’s rise, on the formidable debating skills of its leader, Nigel Farage, seems driven more by disaffection with London politics and immigration than a strong desire to quit the European Union.

But a vote for Scottish independence would change all that. And here the polls are moving in the opposite direction. Until a year ago, Scotland’s quitting Britain seemed a remote possibility.

But in one ICM poll last month, 42 percent of those quizzed said they wanted to stay in Britain, versus 39 percent who wanted to quit. Alex Salmond, the Scottish nationalist leader, is running an effective populist campaign. The odds from the betting house William Hill currently imply a 30 percent chance of Scotland’s exit from Britain. Its departure would increase the chances of a British exit for four reasons.

David Fuller's view

Unusually, there are at least three known and politically seismic events within the next three years that will significantly affect the UK’s development.  Scotland’s independence referendum will take place on 18^th September 2014.  The UK’s next general election will be held on 7^th May 2015.  If Cameron wins that election, he has promised that a referendum on UK membership of the EU will take place by 2017.

Whatever one feels about the outcome of these events, they will certainly create some uncertainty, not least among multinational companies and international investors.