Decked out in campaign posters, Berlin is a city in the midst of poll fever. Full-page advertisements have been bought in the city's newspapers, street protests are being held, and sport and television personalities are being drawn into emotional public debates. But this is no election campaign: at stake is the soul of this devoutly secular city.

Only 36% of people profess to having a religious allegiance in Berlin, often described as the "atheist capital of Europe". So a referendum today on whether religious education should have the same status in the classroom as ethics teaching has provoked a bitter stand-off. Christian Democrats and the churches have railed against the left, believers against atheists, west Berliners against east Berliners, and the middle classes against the proletariat.

Unlike in other German states, secular ethics has been a compulsory topic in the city-state of Berlin since 2006, after the "honour killing" of a Turkish woman murdered by her brother because of her western lifestyle.

But religion remains only an optional course. That has been so since the end of the war, when it was decided that religious teaching should be available in all schools as an answer to the "godless" Nazi crimes. But participation was voluntary, lest authorities were accused of thrusting ideologies on pupils.

The "Pro Reli" campaigners, led by figures from the CDU of the chancellor, Angela Merkel, and church leaders, want to change the regulations so that pupils can choose between ethics and a religion class, with Muslims, Catholics and Protestants being taught separately. "Pupils now have the choice between religion and free time - basically between religion and the ice-cream salon," said Berlin's leading Protestant bishop, Wolfgang Huber. "It is not really fair to say that pupils have a choice ... Religion needs a proper place in the school timetable."

The "Pro Ethik" campaign is being spearheaded by the Social Democrats and the Left party, who make up Berlin's city government. They argue that ethics teaching is central to ensuring integration in Berlin, which is now home to Germany's biggest Muslim community.

"What's so awful about an ethics lesson in which everyone takes part?" asked Michael Müller, regional head of the Social Democrats. "Isn't it a good thing that I learn something about my neighbour's religion, his family and cultural background?"

The country's most famous television presenter, Günther Jauch, a prominent Catholic, has put his name to the Pro Reli campaign, next to the slogan: "In Berlin, we're talking about freedom. Don't say you didn't have the chance to choose."

Jauch, who is most famous for hosting the German version of Who Wants to Be a Millionaire?, said: "Religious education in Berlin is systematically pushed to the edge of the curriculum and discriminated against on the school agenda. Whoever votes 'yes' [in favour of religious lessons] on Sunday, will be voting for the free choice between ethics and religion." Berlin's mayor, Klaus Wowereit, lashed out at Jauch's involvement in the campaign. In a newspaper interview, he said: "I think it's scandalous that intelligent people like my friend Jauch can talk here about freedom. He doesn't know the meaning of the word."

The Pro Reli camp has repeatedly accused the Pro Ethik camp of trying to impose an agenda on school pupils similar to the atheist East German school curriculum; the Pro Ethik camp has in turn blamed the "Pro Relis" for launching a crusade against non-Christians.

Pupils are also divided, although according to polls the younger ones in particular are more in favour of religion. More than 600,000 votes - a quarter of Berlin's electorate - are needed for the referendum to be considered valid.

"I don't know why they just can't combine the two topics, then no one could say there was anything unfair about it," said Benedicta, a 15-year-old pupil at a nondenominational school in Friedrichshain, in east Berlin.