A controversial Democratic Unionist councillor has issued an unreserved apology for a post on Facebook that endorsed a fictional account of a massacre of senior Sinn Fein members.

The DUP has launched an investigation into the actions of Belfast City Councillor Ruth Patterson, branding her comments "completely unacceptable".

Her remarks were in response to a fictional news post on the social networking site which reported that an indiscriminate gun and bomb attack had taken place in Castlederg in Co Tyrone, where a disputed republican march is due to take place this month, with a number of senior Sinn Féin members said to be among those killed.

Commenting on the post, Ms Patterson wrote: "Who cares how we would be judged, we would have done a great service to Northern Ireland and the world..."

She added: “Would I shed a tear? No. Would I loose (sic) a night’s sleep? No, would I really worry about what anyone else thought? No.”

After the comment came to light today, she issued an apology.

“I unreservedly apologise for this lapse of judgement,” she said. “I completely withdraw this flawed Facebook comment. Murder and terrorism can never be justified in any circumstances. To descend to such a level would be to repeat the actions of those we stood against during the dark days of Northern Ireland’s Troubles.

"My ill-judged comment is completely incompatible with the polices of the Democratic Unionist Party and what is expected of me as an elected representative. I recognise that my careless comments fall short of the standards expected of anyone holding a responsible post. I deeply and sincerely apologise."

The DUP also issued a statement about the incident.

“Ruth’s comment was completely unacceptable and not in keeping with the party’s policies or the standard of behaviour required of our members,” said a party spokesman. “Whilst the officers of the party will clearly be looking at this issue they will take account of her subsequent statement.”

Ms Patterson’s controversial post has since been deleted from Facebook.

The former deputy lord mayor of Belfast City Council has courted controversy before. She blamed a hearing problem for failing to shake a Sinn Féin mayor's hand in 2011 following his election.

Sinn Féin MLA Barry McElduff had earlier called on the DUP to act. “This type of talk is totally irresponsible and highly dangerous,” he said.

Originally from Dungannon in Co Tyrone, Ms Patterson came to Belfast in the 1970s to train as a nurse in the Royal Victoria Hospital. In 1982 she joined the Ulster Defence Regiment (UDR) because of what she described as a deep sense of duty to defend her country against the growing IRA threat.

PA