The leaning tower of Bologna is sealed off over fears it is about to COLLAPSE: Officials set up exclusion zone around 154ft medieval structure in 'highly critical' condition

A leaning medieval tower in northern Italy has been sealed off over fears it may soon collapse.

Authorities have started to build a 16ft barrier around the Garisenda Tower to collect debris after the city's council said the situation was 'highly critical'.

The 154ft tower, built 900 years ago, usually sits at a four-degree angle but steady monitoring by the University of Bologna has seen recent shifts in how it leans.

In October, experts warned that the tower's weak foundations might be subsiding, requiring an injection of filler material to add more stability.

And the city's tourist board said they had detected abnormal noise, oscillations, vibrations, and movements of a few millimetres within the tower.

The site of the Garisenda tower was sealed off two months ago after sensors first picked up changes in its tilt.

Inspections of the site then revealed some of the material foundations were wearing away after nine centuries.

The Garisenda Tower and the Asinelli Tower next to it were built between 1109 and 1119, a few decades before the then-town's rapid expansion into one of the main commercial hubs of northern Italy.

The iconic towers are barely younger than the University of Bologna itself, an institution dating back to 1088 and famously the oldest continuously operational university in the world.

The Asinelli Tower lacks a lean and is still open to the public for visitors to climb. But Bologna's tourist board says losing the smaller structure would be a devastating loss to the city's rich history.

'Given the cultural relevance of the Garisenda tower, its hypothetical loss would be tragic, not only tourism-wise but for Bologna and Italy’s history as well,' a spokesperson said. 

In October, the spokesperson said the signals detected so far did not lead them to believe the tower would collapse.

Lucia Borgonzoni, the Italian culture undersecretary, also announced the government would fund work to reinforce the tower using around €5 million (£4.3m) from Italy's EU national recovery fund.

Similar to most of Bologna's medieval towers, the Garisenda Tower was built on a ring-shaped base of mortar terracotta bricks, and river stones.

It is this base material which is the likely cause of the tower's drastic lean as the foundation sinks into Bologna's soft ground.

Even by the 14th century it had begun to lean and was scaled back to avoid collapse.

At 48 metres tall, its only slightly shorter than the medieval Romanesque leaning tower of Pisa, while also boasts an extra degree of slant.

In spite of its structural problems, experts say the structure in Pisa could remain stable another 200 years.