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UKIP's strong showing in the local elections, and possible triumph in the poll to elect members of the European parliament, is an event of economic significance.

Unfortunately I am not sure quite what the impact will be, and that is not just because I am too dim. There remain a number of big imponderables.

That said, UKIP makes the starting point for any analysis pretty simple, because it has just two big and interconnected policies - withdrawal from the European Union, and a curb on immigration, specifically immigration from the rest of the EU.

Now it is very difficult to know whether, in an economic sense, Britain would be better or worse off from EU withdrawal in the very long term. There might be compensation for curbs on our almost frictionless access to the EU's slow-growing but huge market and reduced influence over the development of that market from a removal of costly red tape, the unleashing of entrepreneurialism at home, and a spur to exploit faster growing markets in other parts of the world.

In the short term, of course, it is hard not to see big costs from withdrawal, including a serious investment hiatus as companies big and small agonise about what exit would mean, and relocation of some big companies into a regime less tortured by its EU status.

But whether EU membership is good or bad for us is just guff for current purposes. Because what is unclear is whether UKIP's success makes it more or less likely that the UK will head for the exit.

It is quite plausible that the rise and rise of UKIP, and of other eurosceptic parties all over Europe, will give EU governments such a jolt, such a powerful blast of early morning coffee, that they will embark on the kind of reform agenda that would take the wind out of UKIP's sails.

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That is what the prime minister will be fervently hoping, as he ponders the existential threat UKIP poses to his party, and he thinks about how to pursue his ambition of persuading the rest of the EU to allow the UK greater rights of self-determination as an EU member.

As for UKIP's desire to limit immigration, which resonates with so many, the same economic and political ambiguities apply.

On the one hand, the influx of low-wage, hard-working immigrants - and some high-skilled ones - has probably bolstered our economic recovery by improving corporate competitiveness, and helped the government pay down its excessive debts (because these migrants tend to pay more in tax than they take out in benefits and use of the welfare state).

The presumption in Scotland is that a UKIP triumph in England will bolster the independence campaign

But the benefits of recovery have not led to a recovery of average living standards, precisely because immigration has increased the population.

It is striking that London, where immigration is such a huge element in the economy, was relatively barren ground for UKIP. That may imply the threat to living standards and quality of life of immigration is more in the perception than the reality.

What is probable, however - and will concern many in bigger companies - is that Labour and the Tories will view UKIP's success as proving that the debate on whether immigration is economically positive or negative, has become an irrelevance at best and a danger to their own prospects.

It is very difficult to see how the overnight electoral events will do anything but reduce the openness of the UK to working people from the rest of the world, for better or worse.

And another thing - the presumption in Scotland is that a UKIP triumph in England will bolster the independence campaign, because many Scots see UKIP as a "little England" party.

Scottish independence would also have big economic consequences, north and south of the border - consequences that are as passionately contested as those that would stem from EU exit.

I will be revisiting the big Scottish debate soon enough. But for now the point to note is that Farage's surge is the equivalent of a huge rock having been dropped in the UK economic pond - and the effect of the waves cannot be predicted with certainty.