SDLP MLA Conall McDevitt tonight dramatically quit politics after admitting he failed to declare payments made shortly after he was elected.
In a statement, the south Belfast representative, who was considered a rising star within the party and once challenged for the leadership, said his resignation would take immediate effect.
The payments which amounted to almost £7,000 were made by Mr McDevitt's former employer, public relations firm Weber Shandwick.
He said: "Between March 2010 and August 2010, I received a total of £6,750 from my former employers, Weber Shandwick. These payments related to internal work within Weber Shandwick. I resigned as managing director of the Belfast office in December 2009 and I provided support and mentoring to the new management team following my departure from the company. I was never asked nor did I ever represent any of the company's clients whilst an MLA.
He said failure to declare the earnings, which were paid to mentor the company team that replaced him, had fallen short of standards expected from an MLA.
"These payments were made t hrough JM Consulting, a consultancy which my wife has an interest in. These are registerable interests under the Assembly rules. I have now registered these earnings on my register.
"My failure to register these interests at the time means that I have fallen below the standards expected of me in public office," he said.
Mr McDevitt has represented the affluent south Belfast constituency at the Northern Ireland Assembly since January 2010.
He said it had been an "honour" to have been elected, and claimed that at all times he had tried to discharge his duties as an MLA with integrity.
But he added: "There is no question that I failed to do so on this occasion. I apologise unreservedly for my failures in this regard."
He has come under significant public pressure after it emerged his wife, Joanne Murphy, a Queen's University academic, had received more than £30,000 over the last two years for work as a researcher assisting his role on the Northern Ireland Policing Board.
Ms Murphy's company JM Consulting was also paid more than £14,000 for research and secretarial work from Mr McDevitt's expenses.
SDLP leader Dr Alistair McDonnell said Mr McDevitt had made a positive contribution to the party and his constituency.
However, Dr McDonnell said stepping down was the appropriate action.
"Conall McDevitt has done the right thing by resigning after admitting his failure to register funds received by him during his time as an MLA in accordance with Assembly rules," he said.
"I welcome Conall's speedy and definitive response in this regard and accept his resignation.
"Conall will be a loss to the Assembly party and to south Belfast and I would like to wish him and his family all the best for the future."
Mr McDevitt said he would be taking some time to consider his future.