The SDLP South Belfast MLA Conall McDevitt has announced that he is to quit politics with immediate effect.
Mr McDevitt says he failed to declare more than £6,000 earned from a public relations company shortly after he was appointed as an MLA.
He said he failed in his duty to uphold the standards expected of an MLA.
The money was paid by his former company, Weber Shandwick, for Mr McDevitt to mentor the company team that replaced him after his election.
In a interview with BBC Northern Ireland political editor, Mark Devenport, Mr McDevitt said: "I'll be resigning my seat as an MLA today with immediate effect, I'll be notifying the speaker of my intention to do that.
"In early 2010 I received payment from my former employers. I'd been managing director of a company and I received payment in order to help the new management team bed in.
"That should have been declared and it wasn't. That's a serious breach, in my opinion, of the code.
"I have rectified it today, but I entered politics to try and bring about change and positive change, and I feel I have fallen below the standards expected of someone in public life.
"Therefore I think there's a duty on me to resign my position in public life in order to protect the integrity of the institution and to acknowledge the error that I have made, for which I am obviously deeply sorry."
In a separate statement, Mr McDevitt said: "Between March 2010 and August 2010 I received a total of £6750 from my former employers, 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 to nor did I ever represent any of the company's clients whilst an MLA.
"These payments were made through 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."
Mr McDevitt has represented the South Belfast constituency at the Northern Ireland assembly since January 2010.
He was co-opted to the position, replacing the SDLP MLA Carmel Hanna, and retained the South Belfast seat in the 2011 assembly elections.
In a statement, SDLP leader Alasdair McDonnell said: "Conall McDevitt has done the right thing by resigning after admitting his failure to register funds received by him during his time as MLA in accordance with assembly rules.
"I welcome Conall's speedy and definitive response in this regard and accept his resignation."
Dr McDonnell said the South Belfast MLA "made a very positive contribution not only the constituency but also to the wider SDLP party where he showed energy, enthusiasm and drive".
"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," the SDLP leader added.
On Tuesday, Mr McDevitt confirmed that his wife, Joanne Murphy, received £30,000 in public funds in the last two years, carrying out research for him in his role as a member of the Northern Ireland Policing Board.
Last month, it was revealed that more than £14,000 from Mr McDevitt's assembly office cost allowance was paid to researchers through his wife's firm, JM Consulting.
Sums of £8,000, £4,900, £800 and £600 were paid through JM Consulting to four researchers between 2010 and 2011.
Mr McDevitt said that money was paid to the researchers and his wife did not benefit from the arrangement.
The 41-year-old outgoing MLA is a former director of communications for the SDLP.
He held the post from 1996 to 1999, a period that included the political negotiations that led to the Good Friday Agreement.
He is also a past chairperson of the Chartered Institute of Public Relations in Northern Ireland.
Mr McDevitt was born in Dublin in 1972 and grew up in Malaga, Spain.