Sinn Féin's MEP representing Northern Ireland Bairbre de Brún has submitted her resignation to the European Parliament.
Party President Gerry Adams said Ms de Brún had taken the decision for personal reasons.
Mr Adams said he has worked closely with Ms de Brún, who topped the poll in the 2009 European elections, for over 30 years.
He asked that the media and others respect her privacy.
"She has served diligently and effectively in a variety of roles including head of our International department, a Good Friday Agreement negotiator, an MLA for west Belfast, a Minister in the Executive and latterly as MEP for the six counties," Mr Adams said.
"While Bairbre has resigned from her current position in the European Parliament, she remains a senior figure within Sinn Féin," he added.