Real Madrid president Florentino Perez has admitted the world's richest club buys players who sell shirts.

It's been the open secret of the club's massive commercial success since the turn of the century but in a rare interview given to Columbia Business School professor Steven Mandis - to be aired on the BBC World Service this week - Perez pulls no punches about Madrid's methods.

He says: 'People understand the obvious – that we sign big players like Ronaldo, [Zinedine] Zidane and [Luis] Figo. But after the players there is the economic sports equation.

Real Madrid president Florentino Perez has admitted the club signs players who sell shirts

Perez is pictured alongside David Beckham on the day the Englishman signed for Real in 2003

'In our model it is fundamental to understand that in the signing of every player, we have to have in our heads: what's the income generated?'

Madrid's president, who will clock up 500 games at the helm this coming weekend, has been criticised for signing players for their commercial worth and not just their suitability to the needs of the team.

He signed Europe's best right-sided midfielder David Beckham in 2003 despite the fact Real already had Luis Figo.

But Beckham's huge value to Madrid off the pitch and his ability to sell shirts and open up the Asian market to the club made his signing a runaway financial success, despite the fact the team went three years without a trophy after he arrived.

In the interview, which features as part of Mandis's book 'The Real Madrid Way: How Values Created the Most Successful Sports Team on the Planet', Perez admits: 'In the year 2000, Real Madrid suffered a very big economic crisis.

Perez, who will rack up 500 games as president, is pictured with Zinedine Zidane in 2001

Brazilian superstar Ronaldo (centre) poses with Perez and Real legend Alfredo di Stefano

'So big that it risked losing its status as a member-owned club. But the club has to belong to its members and not belong to just one individual.'

Perez argues that without a wealthy private backer the club has no option but to consider the marketing worth of every new signing.

It has not always been a popular modus operandi with managers. Last year, former coach Carlo Ancelotti called the signing of 16-year-old Martin Odegaard and the pressure to play him a 'public relations move'.

And earlier this week, former Real coach Vicente Del Bosque's right-hand man Toni Grande suggested Perez had given the order for Fernando Morientes to be left out of the 2002 European Super Cup final.

Grande said: 'An hour before the game against Feyenoord Jorge Valdano came in and said to Vicente: "Tell Fernando [Morientes] to not get changed because he is being transferred. It's an order from the president".'

But it was the influx of the so-called Galaticos such as the Brazilian Ronaldo – who replaced Morientes in the team – that helped Real Madrid overtake Manchester United in the football rich list.

Through conversations with fans, players, coaches and board members – including Perez – Professor Mandis lifts the lid on Real Madrid's sporting and financial success.

The documentary 'Inside Real Madrid' is on the BBC World Service on Wednesday, February 8 at 7.30pm GMT.