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A medieval tower in the Italian city of Bologna that leans by as much as its famous counterpart in Pisa has been sealed off over fears it may collapse.

Authorities have begun constructing a 5m (16ft) high barrier around the 12th Century Garisenda Tower to contain debris in the event that it falls.

The 47m (154ft) tower tilts at a four-degree angle, and monitoring has found shifts in the direction of the tilt.

The city council said the situation was "highly critical".

The Garisenda Tower is one of two towers that dominate the skyline of Bologna. The other, the Asinelli Tower, is around twice the height and also leans, though not so dramatically, and is usually open for tourists to climb.

The structures were built between 1109 and 1119, though the height of the Garisenda was reduced in the 14th Century because it had already begun to lean. The tower is mentioned in Dante's poem The Divine Comedy, which was completed in 1321.

The site was first closed in October after sensors picked up the changes in the Garisenda's tilt and inspections revealed deterioration in the materials that make up its base.

The council has launched what it calls a civil protection plan to preserve the tower and said the work now being started "represents the first phase of making it safe".

It said that as well as containing debris, the barrier would protect surrounding buildings and people in the event of a collapse. It said metal rockfall nets would also be installed around the tower.

Construction of the barrier will be completed early next year, while the tower and the plaza beneath it are expected to remain closed for a number of years while restoration work is carried out.

The city estimates that the barrier alone will cost €4.3m (£3.7m) and has launched a crowd funder to pay for the restoration.

It called the project an "extraordinary challenge" that would require "commitment from the entire city and from those all over the world who love Bologna and one of its most important symbols".