A Sinn Féin assembly employee in Northern Ireland has resigned after admitting involvement in an incident where a portrait of a former DUP mayor was taken off the wall at Belfast City Hall and damaged.

The party, which has been rocked by a number of recent controversies north and south of the border in Ireland, said police had been informed and the staffer suspended. It later said they had resigned.

The DUP said the portrait was removed from a wall and the glass in its frame smashed during an event on Saturday evening to celebrate an Irish language group’s 20th anniversary. The portrait of Wallace Browne was painted by the Jewish artist Israel Zohar, it said.

In a social media post, the DUP leader, Gavin Robinson, said: “We don’t know if the motivation was sectarian bigotry, antisemitism, wanton destruction or a heady mix of the three … but [it’s] a disgrace.”

The Police Service of Northern Ireland said that at 4.50pm on Monday, they had received a report of criminal damage to an artwork in Belfast City Hall.

It piles the pressure on a beleaguered party which has seen four high-profile resignations in recent weeks.

In a statement released on Monday evening, a Sinn Féin spokesperson said: “Today, 21 October, a Sinn Féin employee, who works in the assembly, made the party chief whip aware of their involvement in an incident regarding a portrait in Belfast City Hall which took place on Saturday 19 October.

“The employee was immediately suspended, and we have notified the PSNI today. The employee has now resigned from their employment and their party membership.”

The incident comes just days after revelations that a former Belfast mayor, Niall Ó Donnghaile, had quit Sinn Féin last year after the party received complaints that he had sent inappropriate texts to a teenage party member.

Three weeks ago, two press officers also resigned after they gave a reference to another senior press officer who has admitted a series of child abuse offences.

Last week one of its most respected TDs in the Dáil, Brian Stanley, also quit over allegations of gross misconduct, which he said had been handled in an internal “kangaroo court”.

With a general election in the republic due to be held in November or December, opposition parties are exploiting Sinn Féin’s difficulties for maximum electoral gains.

A Sunday Times poll over the weekend showed the party dipping to 16% support from a high of 30% two years ago.