A former pro-Remain MP and unionist from Northern Ireland will not contest the General Election.
Lady Sylvia Hermon, 64, said she would not be defending the North Down seat she first took in 2001.
She was under pressure from a Democratic Unionist rival who cut her majority to 1,208 in the last poll in 2017.
Lady Sylvia said: “Serving as an MP is a tremendous privilege, and I remain profoundly grateful to all those who placed their trust and confidence in me in the last five general elections.
“It has undoubtedly been the greatest honour of my life to serve the people of North Down as their Member of Parliament since 2001, and so it is with enormous sadness that I have decided not to contest the next General Election in December.”
Lady Sylvia is a former Ulster Unionist who was married to the late Royal Ulster Constabulary (RUC) chief constable Sir Jack Hermon.
She was a liberal voice at Westminster and was resolute in her opposition to a no-deal Brexit.
She represented one of Northern Ireland’s most affluent and staunchly unionist constituencies, made up largely of Belfast commuter towns and coastal villages.
Lady Sylvia added: “This has been a particularly difficult decision but, after much thought, I have concluded that it is the right decision for my family and for me at this time.
“During my 18 years as an MP, I have tried hard to balance public and family responsibilities.
“I owe my family a huge debt of gratitude for allowing me, without ever a word of reproach, to spend so much time at Westminster; they understood how much joy and satisfaction I had from working on behalf of others.
“Now, however, my priorities for the next few years are to spend my time at home in Northern Ireland to see more of my family and to step back from the front line of public life.”
She apologised for any disappointment caused to constituents who had urged her to stand again for re-election and thanked volunteers in previous election bids.
“If my successor enjoys the role of MP for North Down half as much as I have done, a smile will constantly be on his or her face.”
She entered politics in 1998, joining the Ulster Unionist Party due to its role in the Good Friday Agreement negotiations which largely ended decades of violence.
In 2005, she was the only Ulster Unionist MP to retain her seat after the party suffered catastrophe in the Westminster election.
Lady Sylvia left the UUP in 2010 in opposition to the party’s short-lived alliance with the Conservatives.
She has been an independent since then and an assiduous worker on the Northern Ireland Affairs Committee, where she has scrutinised official performance with a keen eye for detail.
She recently called for a clear guarantee on the new Brexit deal.
Lady Sylvia has highlighted unionist anger at Prime Minister Boris Johnson’s proposed agreement with the EU over its perceived implications for the constitutional status of Northern Ireland.