British Prime Minister Theresa May attends her last European Union summit today before she triggers off formal negotiations to leave the Union.

In all future summits, the British politician will be a half-outsider, a leader of a country with one foot out of the door.

But although the heads of states and governments of the EU's 27 remaining nations will pretend today that, notwithstanding Brexit- as the process of Britain's separation is now known - their Union remains strong, the reality is that the tussle about the future shape of the EU has now been unleashed.

And it will be an acrimonious debate - when Mrs May leaves the two-day summit early tomorrow, the other European leaders will immediately be locked in bitter disputes about future EU decision-making procedures and senior bureaucratic appointments.

Brexit has changed Britain; it's about to change Europe as well, and not necessarily for the better.

Soon after the British narrowly voted in a referendum last June to leave the EU, European leaders agreed that they would refuse any preliminary negotiations with the government in London until the British formally announced their intention to withdraw, as required by current EU treaties.

This stance worked well for the EU. It put the burden on Britain to say what it wanted and allowed EU officials to prepare their positions.

Most importantly, it prevented the Brits from trying to split the Europeans by offering separate deals to individual countries.

The snag is that the blanket refusal to deal separately with Britain also papered over big European cracks which are now about to be revealed.

There is no precedent for handling such negotiations. The EU and its individual governments will have to make up procedures as they go along, and tensions are certain to arise.

The chief reason for predicting such tussles is that, despite appearances, the EU remains profoundly split about how to deal with Britain.

All 27 remaining member-states agree that the British should not be allowed to "cherry pick" between obligations and advantages in any future relationship they negotiate with the EU.

But they disagree over what this means. For the older, richer western EU nations, the key to the Brexit negotiations is to ensure that Britain's departure does not affect the future operation of the Union, and that the British should be worse off outside the EU, so as not to encourage other countries to follow the British model.

But for the poorer and newer members of the EU from central and eastern Europe, the key objective is to keep Britain engaged as much as possible.

This is partly because Britain's superior military capabilities are important in the current tensions between Europe and Russia, but also because Britain is home to many East European migrants, and especially about a million Polish citizens.

Last month, Mr Jaroslaw Kaczynski, who runs Poland's ruling Law and Justice party and is the country's most powerful politician, said: "Everyone loves the United Kingdom and doesn't want to give it a hard time."

Mr Kaczynski's shadow looms large over today's summit, for the Polish leader is objecting to the reappointment of his fellow countryman Donald Tusk as president of the European Council, the EU's highest-ranking official. Other EU leaders, who remain adamant that Mr Tusk's term should be renewed, are now left in the extraordinary position of going against the government of the home country of the EU president.

But a far bigger dispute looms over plans promoted by France to create a core of powerful countries within the EU once Britain leaves. French President Francois Hollande hosted a special summit on the matter last week, attended only by the leaders of Germany, Italy and Spain.

Mr Hollande claims that "a Europe of 27 can no longer be a uniform Europe", and wants a number of big countries to move forward by forging a tighter Union, while leaving others with an unenviable choice of either accepting what the bigger nations are dictating or being relegated to the peripheries of the continent.

In the past, Britain could be relied upon to block such moves. Now, however, it falls to other, less powerful nations to do so.

For some European leaders, the fact that Britain is about to embark on its exit is a welcome development.

Britain has been a difficult EU member.

But many other European nations will discover at today's summit that, next to a difficult Britain inside the EU, the worst alternative for them is an EU without Britain.