British prime minister Theresa May has decided to call an early election, before the practical outworking of her Brexit strategy becomes obvious to voters. She wants to be free to modify her strategy, and to do so she needs a bigger parliamentary majority.

She claims otherwise. She says she is calling the election because opposition parties oppose her Brexit strategy. They don’t oppose it, actually. The only opposition party that opposes her strategy outright is the Scottish National Party (SNP), which takes that position because that is the way Scotland voted in the Brexit referendum.

In any event, the SNP could not bring down May’s government on Brexit unless Labour, the Liberal Democrats and – most importantly – a significant number of the prime minister’s own Conservative MPs voted with them, which is not at all likely to happen.

Rather more bizarrely, May justifies her call for an immediate general election on the grounds that the Labour Party has threatened to vote against the final agreement she may come back with in two years’ time. What does she expect? That the main opposition party would give her a blank cheque on the terms of Brexit?

Usually negotiators actually find it useful to be able to say, when looking for a concession, that if they do not get it, the overall deal might be opposed in parliament . If she is to be believed, Theresa May apparently wants to give up that negotiating chip.

She ostensibly defends the sovereignty of the UK parliament. But now she is calling an election because the opposition will not promise not to exercise their sovereign parliamentary rights.

My own sense is that the reasons she has advanced may not be the real ones for which she has sought an early election. She is seeking an election to increase her overall majority so she will no longer be dependent on a hardcore group of around 60 Euro-hostile Conservative MPs who hold disproportionate power because the Conservative overall majority is so small. For these MPs, hostility to the EU has become a religion, a religion which brooks no argument, and a religion for which any economic sacrifice can be justified, even the sacrifice of the livelihoods of their own constituents.

May does not want to find her day-to-day negotiations with the rest of the EU subject to the whim of these people, by whom the slightest compromise with the EU 27 will be portrayed as a betrayal.

It is important to remember that May, like the rest of her party, has never taken much interest in how the EU works, in its procedures and rules, and in the compromises that underlie its very existence. She has this in common with many politicians in bigger European countries who treat the EU as a sideshow to national politics.

So, even though her party sponsored the idea of holding a referendum on leaving the EU, she did not give much thought to what that might mean until the last few months, when it suddenly became something that was going to happen. In a sense, she and her party are now finding out a lot about the EU for the first time, just as they are leaving it.

Her first reaction to the referendum was to get her party behind her, so she told the Conservative Party Conference last year that she would go beyond the mere terms of the referendum. She would not just leave the EU – she would refuse to join the European Economic Area and also refuse to join the customs union. This hard line bought the temporary quiescence of the Euro-hostile MPs, up to and including on the terms for triggering of Article 50. But now come the actual negotiations.

This is where May’s rhetoric meets the reality of a rules-based international trading system, where unpleasant compromises are essential if you are to persuade others to open up their markets to your exporters, to your bankers, to your planes, and to your people.

In a rules-based international trading system, you cannot unilaterally make, interpret and enforce the agreed rules in a way that suits only you. There has to be a common system that involves some concession of sovereignty. You often have to accept an external enforcer, like the European Commission. And you often have to accept an external body interpreting the meaning of the rules, like the European Court of Justice.

This is unacceptable to those who have made national sovereignty a religion. It is unacceptable to some of May’s Euro-hostile MPs, and also, incidentally, unacceptable to some of the supporters of US president Donald Trump.

I have been reading publications of Conservative-supporting think tanks, such as the Bruges Group and Leave Means Leave, and they are now discovering how costly it will be for the UK to leave the EU customs union. The UK will have to introduce customs controls on the goods bought and sold between the UK and the EU. This will involve checking where the goods come from, if they are properly labelled, if they are safe, and if the tariffs due have been paid. The delays will be horrendous. Customs clearance alone will add 8% to the cost of goods arriving by sea from Ireland or the rest of the EU.

At the moment, 90m customs declarations have to be checked in the UK for goods from outside the EU. Once the UK leaves the customs union, its customs officials will have to check 390m documents. The UK will not only exclude itself from duty-free access to the EU market, which represents over 50% of UK trade, but it will also lose the benefit of trade agreements the EU has negotiated with 60 other countries, which account for a further 17% of UK exports.

Prime minister May is also beginning to discover that her hard line on immigration will have costs too. Twenty per cent of employees on UK farms – and 29% of employees in food processing plants – are EU nationals who will lose their right to live and work in the UK. When the UK tries to negotiate trade deals with countries outside the EU, like India, it will find that it will face demands for more Indian migration to the UK.

UK airports will find themselves losing business when the UK has to leave the EU Open Skies Agreement with the US. More US transit traffic will be routed through Dublin. The UK will also have to try to join the European Common Aviation Agreement as a separate member if UK-owned airlines are to have the right to fly passengers between EU airports. Rivals will not make it easy.

UK farmers and food producers will find themselves facing tariffs of 35% on dairy exports, 25% on confectionery, and 15% on cereals. UK lamb production will be hard hit.

If Theresa May wants to be able to make deals to avoid some of these bad outcomes, she will need the sort of flexibility that her Euro-hostile backbenchers would not allow her. That is why I think she is calling a general election now.

The strategy may backfire. If, during the election, she is forced into explicitly ruling out various possible compromises with the EU, she will end up with less flexibility than she currently enjoys.

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