By Tim Franks BBC Europe correspondent in Berlin

In a year of change for the European Union, with the recent addition of 10 new countries and agreement on a new constitution, its leaders now face another big decision.

The entry of Turkey into the European club may be, in some ways, their biggest decision - to allow in a country that is arguably more Middle East than Europe.

Would EU rejection strengthen the influence of Turkish Islamists?

But while Turkish Prime Minister Tayyip Erdogan's visit has gone down well in Brussels, there is strong opposition in many European countries.

And the entry negotiations could go on for about 15 years, officials say.

At the literature house in Berlin, a crowd gathered for a talk on Islamist terrorism, by one of Germany's top politicians - Friedbert Pflueger.

The interior ministry has said that one sure way to increase disaffection in the Muslim world about the West is to rebuff Turkey's advances towards the European Union.

Mr Pflueger, the parliamentary spokesman on foreign affairs for the opposition Christian Democrats (CDU), disagrees.

"Letting Turkey in only makes sense if you are British and want an end to further integration of the European Union. In that case it is a brilliant idea," he says.

If we really anticipate full membership of Turkey I see it rather around 2018 or 2020 than anything else Ulrike Guerot, European foreign policy specialist

"Then we are further away from a single currency, further away from a state-like character for the union.

"We don't want to be a Christian club. But we want to have an identity as Europeans and, of course, to take such an enormously big country, with such enormous problems and with basically another religion - that poses problems to the coherence of the EU."

Germany has itself absorbed more people of Turkish origin than anywhere else in the EU - around 2.5 million. Many of them live in the Kreuzberg district of Berlin. But while most Germans appreciate the Turkish presence here, the latest opinion poll suggests a majority against Turkish membership of the EU.

Ramazan Aygar, a Turkish lawyer working in Kreuzberg, warns that a decision not to let Turkey join the EU soon could send the country spinning eastwards.

"Turkish people will say 'OK we don't want to go West, we are going to our brothers, our Muslim brothers in Iraq, Iran, Syria'. I think it is a problem", he said.

EU identity problem

The decision on letting Turkey start final membership negotiations lies first with the EU Commission in two weeks' time and then with the EU member states in December.

In public, the 25 EU leaders may be committed to Turkey joining, but European foreign policy specialist Ulrike Guerot says there is privately deep concern.

HAVE YOUR SAY Turkey should stay independent and try to play a key role within the Muslim world Martin Schuschnigg, Hanover Send us your comments

"If we really anticipate full membership of Turkey I see it rather around 2018 or 2020 than anything else," she said.

Coming to Berlin you are surrounded by symbols of a Europe transformed. Standing outside the Reichstag of a Europe that has gone through the horrors of fascism and the physical division of communism, the transformation is not complete.

In considering what to do with Turkey, the EU has to think about itself. Whether it is there to be a tight political entity, to be a counterweight to the US, to be a Christian club or to be primarily just a free trade area.

The decision about Turkey will tell us about that country, it will also tell us a lot about the European Union.