ST Helens MP Shaun Woodward stepped down from the shadow cabinet last week.
Mr Woodward, the MP for St Helens South and Whiston, said he was convinced that “now is the right time for others to take forward the next stage of work in Northern Ireland”.
In his resignation letter, Mr Woodward also told how it had been “a great privilege” to work as Northern Ireland secretary for three years in the last Government, and in the shadow role for the last 18 months.
He wrote: “I am especially grateful to all those in the Labour Party and the political parties of Northern Ireland who put their trust in me. The generosity and kindness of the people of Northern Ireland has been extraordinary throughout - I am forever in their debt.
“I would also like to thank my constituents in St Helens who have been so understanding and supportive throughout my time on the front bench. I will, of course, continue to do all I can to serve their interests and to work hard protecting all those who live and work in St Helens.”
In response to Mr Woodward’s resignation letter, Ed Miliband wrote: “I would like to thank you for your contribution both in Government and in opposition and the support you have offered me, particularly in the past year.
“As Northern Ireland secretary you served with integrity, taking forward the peace process with skill and diplomacy. And in opposition you have continued diligently to serve the people of Northern Ireland and represent their interests.”
Following Mr Woodward’s return to the backbenches, Vernon Coaker was named the new shadow Northern Ireland secretary.