Sri Lanka's election commission said it would censor a state-owned TV station in an unprecedented move after accusing the network of bias against the former president's brother, who is standing in upcoming polls.

The surprise move came after the Independent Television Network (ITN) aired a programme alleging loyalists to the previous regime had thwarted a corruption probe into strongman ex-president Mahinda Rajapakse's family.

The broadcast had harmed opposition frontrunner Gotabhaya Rajapakse — brother of the former leader — according to the election commission.

The channel will be banned from showing political content from Monday without the commission's approval, chairman Mahinda Deshapriya said.

"ITN cannot broadcast any political content without clearing with the Election Commission until the conclusion of the election on November 16," said a letter sent from Deshapriya to the network late Saturday and seen by AFP.

The letter also said that any live events filmed by the station must be shown to the commission for approval before being transmitted.

It is the first time that a Sri Lankan election chief has censored a TV station.

The commission said it had received many complaints about other channels campaigning for Rajapakse without declaring their bias, but could not take action as these were all private networks.

Gotabhaya Rajapakse was defence secretary for a decade until his brother Mahinda was voted out in 2015.

The opposition candidate is the current favourite, even though his bid is widely seen as a front to get Mahinda —barred from contesting the election because of a two-term limit — back into power.

Mahinda's rule was marked by the end of a bloody civil war against ethnic Tamil rebels, and an international outcry over alleged human rights violations by Sri Lankan troops.

The election has attracted a record 35 candidates, with nearly 16 million people eligible to vote.

Last week, the commission said it was working with Facebook to take down posts that defamed election candidates.