Reuters_ Work to prevent the collapse of a medieval leaning tower in the heart of the northern Italian city of Bologna will cost 20 million euros ($21.5 million) and take 10 years at least, its mayor said on Wednesday.

The 12th century Garisenda is one of Bologna's two towers, measuring 48 metres and standing next to the taller Asinelli (97 metres). It is a much loved landmark, mentioned in Dante Alighieri's Divine Comedy.

Like the more famous Tower of Pisa, it has leaned for centuries, as the ground on which it was built gave way soon after construction. The area around it was cordoned off last month, due to rising expert concerns about collapse risks.

"I think we will spend no less than 20 million euros, maybe more" to restore and consolidate the tower, Mayor Matteo Lepore said in a press conference at Bologna city hall, as quoted by the ANSA news agency.

"For the Tower of Pisa, it took 10 years for the intervention and the (restoration) project. We have no reason to say it will take us less," he added.

Italy's civil protection agency has put out a "yellow" alert for the area around the two towers, meaning it is under watch, but is thought to pose no immediate risks for people's safety. There are two possible higher alert levels, "orange" and "red".