A motorway bridge built in the 1960s has collapsed during heavy rainfall in Genoa, Italy, killing at least 37 people.

The concrete Ponte Morandi structure, designed by prolific Italian engineer Riccardo Morandi, disintegrated at 11.30am local time on 14 August 2018.

Up to 35 vehicles plunged down a 45-metre drop into a flooded stream below, after a tower and a 210-metre section of the bridge came down.

Italy's interior minister Matteo Salvini confirmed the death toll of 37, although it may still rise. A number of people were seriously injured and up to 12 are still missing.

Cause of collapse unknown

There is speculation that lighting may have struck the stays, or a landslide could have destabilised the base.

Rescuers are still searching through the rubble with sniffer dogs to try and find survivors of the disaster.

Railway tracks running under the bridge have been blocked, and a warehouse sustained damage.

The accident happened just before the Italian public holiday of Ferragosto.

Morandi design unusual at the time

The Ponte Morandi bridge, named after its designer Riccardo Morandi, is part of the Polcevera viaduct and was a key section of Italy's A10 motorway.

Before the accident, the bridge spanned 1,102 metres, supported by three reinforced concrete piers.

Morandi was known for his unconventional cable-stayed bridges that featured an unusually low span-to-stay ratio. As at the Morandi bridge, he often used prestressed concrete, not steel, for the stays.

A bridge he built in Venezuala in 1957, called the General Rafael Urdaneta Bridge, collapsed in 1964 when a fuel tanker crashed into one of the spans.

Bridge had undergone extensive repairs

Since the 1970s the bridge has undergone near-constant restoration work. Initial miscalculations regarding the concrete viscosity caused the vehicle deck to warp, requiring straightening and levelling work.

In 2016, Antonio Brencich – a reinforced concrete construction specialist and professor at the University of Genoa – told Italian media company Primocanale the bridge was "a failure of engineering" and needed to be replaced.

Reinforcement work was undertaken to shore up the bridge's foundations in the same year, but a complete replacement was deemed too disruptive.

Italy's infrastructure spending down

Infrastructure spending in Italy has dropped significantly since the 2008 financial crisis, according to the BBC, remaining lower than other European countries such as the UK, France, Germany and Spain until 2013.

Italian officials have maintained the bridge was properly maintained. According to the Independent Italy's governing party, the Five Star Movement, dismissed fears about the Morandi bridge's instability as a "fairy story", while they were in opposition. The statement has now been deleted.

Italian newspaper Corriere Della Sera has reported that this is the fifth bridge collapse in Italy in as many years.