MP for Gedling and shadow secretary of state for Northern Ireland Vernon Coaker has joined 11 other members of the shadow cabinet and resigned.
The resignations follow the sacking of shadow foreign secretary Hilary Benn by Labour leader Jeremy Corbyn on Saturday night.
The MP released a statement saying he believes it is “time for the party to unite behind a new leader” and serve as an “effective opposition”.
He adds: “I have sought to make it my priority to provide reassurance to the people and political parties of Northern Ireland that Labour remains committed to the principle of consent, as expressed through the Good Friday Agreement and subsequent agreements.
“I know my successor in this role continues this vital work.”