Officials are investigating whether City Council Speaker Melissa Mark-Viverito violated city rules by accepting free consulting work from a lobbyist during her campaign for the top council spot, sources said.

The Conflicts of Interest Board probe stems from a complaint made by the watchdog group Citizens Union over pro-bono work Mark-Viverito received in late 2013 from The Advance Group.

City employees are barred from accepting gifts valued at $50 or more, which includes money, services or other forms of compensation.

A Conflicts Board official said the agency is forbidden from confirming or denying the target of its probes.

A spokesman for Mark-Viverito referred questions on the probe, which was first reported by Crain’s Insider, to her campaign.

“Any step in running the campaign, which included assistance from volunteers, setting up a committee, was done in reliance on campaign advisers and counsel,” said a campaign spokesman.

Officials at The Advance Group wouldn’t comment on the outfit’s role in the potential violations.