The contract will include an option for a three-year extension, which is intended to incentivise investment.

Sbase will be responsible for setting service standards, defining enhancements to the network, and setting fares.

Technical offers for the concession will be opened in July, followed by economic bids in August. Sbase plans to sign the contract in October.

The city says that ridership on the network has increased significantly since it took over the network, rising from 900,000 passengers per day in 2013 to 1.2 million in 2017, while passenger satisfaction rose from 33% in September 2012 to 68% in November 2017.

The network expanded from 47.5km in 2013 to 54km in 2017, and 401 new metro cars have been introduced, 52% of them with air-conditioning. Average weekday headways have also fallen from 4.06 minutes to 3.14 minutes.