The candidate's killing comes ahead of Iraq's parliamentary elections, scheduled for November 11.

"A bomb exploded under the vehicle of Safaa al-Mashhadani, a current member of the Baghdad Provincial Council and a candidate for parliament. He died instantly and three of his bodyguards were seriously wounded," the source, who requested anonymity as they were not allowed to brief the media, told AFP.

The source explained that "the incident occurred at dawn on Wednesday in Tarmiyah", located 40 kilometres north of the capital and part of the Baghdad province.

Mashhadani was running with the Sovereignty Alliance, one of Iraq's largest Sunni Muslim coalitions, led by businessman Khamis al-Khanjar and parliament speaker Mahmoud al-Mashhadani.

The coalition condemned the "cowardly crime", calling it "an extension of the approach of exclusion, targeting and treachery pursued by the forces of uncontrolled weapons and terrorism, all of which seek to silence free national voices".

According to the coalition, Mashhadani "fought and struggled for his people and his city of Tarmiyah, against both terrorism and the forces of uncontrolled weapons".

Prime Minister Mohammed Shia al-Sudani ordered an investigation opened into the attack and called for the perpetrators' arrests, a security source told AFP.

"The entrances and exits (to the crime scene) are currently closed and security forces are deployed," the source added.

Britain's ambassador to Iraq, Irfan Siddiq, condemned the killing.

"Violence against political candidates undermines Iraqi democracy," Siddiq wrote on the embassy's social media in Arabic.

"We stand with Iraq's leaders in condemning this violence."

The majority of Iraq's 329 lawmakers represent Shiite parties aligned with neighbouring Iran.

The upcoming elections are the sixth since the 2003 US-led invasion of Iraq, which toppled longtime ruler Saddam Hussein.