Faleh al-Radhi, the head of the security committee at Babil provincial council, said “the attack was carried against a checkpoint at the northern entrance to Hilla.”
A doctor at Hilla hospital put the number of people killed by the blast at 47, including around 20 members of the security forces, and said at least 72 people were also wounded.
Radhi and police officers confirmed the casualty toll, the heaviest from any car bomb attack in Iraq this year.
Officials said the vehicle was a truck packed with explosives and was detonated after being pulled over by checkpoint security as it tried to enter Hilla.
Pictures posted on social media showed vast destruction around the checkpoint, where cars are usually bumper-to-bumper at that time of day, queueing to be checked by security personnel.
A doctor at Hilla hospital said at least 11 of the wounded were in a very serious condition.
The Islamic State group, which carries out nearly all such attacks, has not had fixed positions south of Baghdad since security forces and allied militias began their fightback against the jihadists in late 2014.
A March 2014 suicide bombing at a checkpoint on the outskirts of Hilla, 80 kilometres (50 miles) south of the capital, killed 50 people and wounded more than 150.
When Iraqi forces began their counter-offensive against IS in late 2014, securing the Shia shrine cities of Najaf and Karbala, south of Baghdad, was a priority.
The jihadist group has been losing territory in Iraq for almost a year. In the most recent operation, Iraqi forces are retaking areas west of the city of Samarra.
In the cities the group retains control over, internal tension appears to be on the rise and the lack of supplies is taking its toll.
Observers have warned that, as their self-proclaimed “caliphate” shrinks towards extinction, IS fighters are likely to revert to their old guerrilla tactics and ramp up suicide car bomb attacks on civilian targets.