El Clásico has long been a proxy war in a divided land and tomorrow's fixture in Spain's Primera Liga – Barcelona host Real Madrid in Camp Nou at 6.50pm – has added spice, where not if necessary, as the clamour for Catalan independence grows stronger.

On September 11, which Catalonia celebrates as its national day, the city was brought to a standstill as protesters, numbered at 1.5 million according to local police reports, rallied for secession from Spain. The slogan for their march was "Catalonia: a new European state".

Pep Guardiola, Barcelona's iconic former coach, who stepped down in the summer after winning 14 trophies in four seasons and is currently on sabbatical from the game in New York, brought a huge cheer when his video message was broadcast during the rally. He held up a green card which read: "Here's one more vote for independence."

As coach, Guardiola used to speak in Catalan during press conferences. Seven million people speak Catalan, which is closer to French than it is to castellano, Castilian Spanish. Dismissive madrileños call Catalans polacos, or Poles, because of the alien, slurping sound of their language. There is a clause in player contracts at Barça which states that foreign players, such as the Argentine Lionel Messi, must embrace the Catalan language.

Catalonia is the biggest generator of wealth in Spain. Its (dwindling) taxes subsidise much of the rest of the country, as Spain struggles with 25% unemployment, the highest in the eurozone. The region, by its own estimates, transfers as much as 9% of its GDP, approximately $22bn a year, to other regions such as Andalucía in the south.

A month after stepping down as president of Barça in June 2010 – the team won two Champions League titles in his seven-year reign – Joan Laporta set up a political party, Catalan Solidarity for Independence, which won four seats in Catalonia's 135-member parliament in November 2010.

"From the beginning of the 20th century, Barça has always been considered the club of Catalonia," he says. "Why? Because the board of directors in a general assembly supported the Estatut de Catalunya [a statute which defines its self-governing rights] in the 1930s and, during my presidency, in 2006, we supported the new Estatut de Catalunya. Since democracy, Catalan rights are not being promoted and, if we don't promote our rights, who is going to?

"Of course under the dictatorship, the repression was more evident. The Catalan language was forbidden. But today we are in the same process. Barça is a representative of Catalan culture, a symbolic church for our country. It is a way to promote our feelings, the best possible tool to promote the image of Catalonia to the world."

Not many people outside Catalonia appreciate Laporta's politics. Few around Spain – outside the Basque Country – sympathise with Catalonia's push for independence, which is so ardently promoted by Barcelona. It offends their notion of a unified nation. In 2005, for example, there was a damaging campaign to boycott cava, the sparkling wine synonymous with Catalonia, when it refused to back Madrid's bid to host the 2012 Olympics.

There are roughly 1 million Real Madrid supporters in Catalonia. They resent the overbearing separatist politics of Barça, particularly when it leaks into the school curriculum.

"In a lot of public schools in music class, the Barça anthem is taught to the children as if it was a Catalan folk song," says author and journalist José Miguel Villarroya. "This is a problem. It's the anthem of the Barça team, like the German anthem or the British anthem. It's not a traditional Catalan song. People in Catalonia support other teams than Barça, like Espanyol and Real Madrid.

"Now there is an agenda. A lot of politicians, bankers and industrialists are trying to create a homogenous society. They say a good Catalan person loves Catalonia. Barça is the image of Catalonia. It is a great team. It wins a lot of trophies. The most important thing in Catalonia is Barça. It's the central column of this feeling. So if you're not from Barça, you're not a good Catalan. If you're from Real Madrid, you're the devil incarnate. Real Madrid is the foe, not only against Barcelona but also against Catalonia.

"For these people, not all Catalans, Real Madrid represents the old orthodoxy, old Spain. It represents the old enemies – Franco's time, centralism and restrictions on Catalans' liberties. This makes it more difficult to be from Real Madrid at the moment. The resentment towards Real Madrid supporters is stronger now. They say, 'Are you a Real Madrid fan? Well, then you are a fascist.' When I was a child in Franco's time, this was not a problem because there was no democracy. People couldn't talk about politics. Real Madrid, Barcelona, Espanyol were only football teams."

The interesting thing about tomorrow's game – outside the fact that last season's champions Real Madrid seek to close an eight-point gap with their rivals – is that it could be one of the last league fixtures between the sides. If Catalonia wins independence – and it is a big if despite some polls last month putting support at 51% – Barcelona, according to governing body UEFA's rules, would have to leave the Spanish league and play in the semi-professional Catalan league.

Richard Fitzpatrick is the author of El Clásico: Barcelona v Real Madrid, Football's Greatest Rivalry, which is published by Bloomsbury