Sinn Féin has said a video of one of its MPs with a Kingsmill-branded loaf on his head on the anniversary of the Kingsmill massacre was “inexcusable and indefensible”.
Party chairman Declan Kearney has said West Tyrone MP Barry McElduff had fallen well short of the standards Sinn Féin expects of its members.
Mr McElduff has apologised for the post, insisting it was not meant as a reference to the republican murders of 10 Protestant workmen in 1976.
He said he “had not realised or imagined for a second any possible link between product brand name and Kingsmill Anniversary”.
Speaking on radio, Mr Kearney said: “What has happened is absolutely inexcusable and indefensible and the party is taking this matter very seriously indeed.”
He added that Sinn Féin wished to express “deep and sincere regret”.
“The reality is huge offence has been caused and I and Sinn Féin strongly disapprove of what has happened.”
Barry McElduff has been summoned to a crunch meeting with the Sinn Féin leadership in Northern Ireland on Monday.
He has also offered to meet the Kingsmill families following the video he posted on social media which he removed on Saturday.
However, the sole survivor of the 1976 atrocity, Alan Black, has described the Sinn Féin MP’s actions as “dancing on the graves of the victims”.
Mr Black said it was “done deliberately to cause the maximum hurt” and added that he would not meet with Mr McElduff.
The video has been widely condemned by local politicians.
Ulster Unionist Party Leader Robin Swann said his party had referred the incident to the Parliamentary Standards Committee.
Meanwhile, the PSNI said they had received a number of complaints which they are investigating.