Nobel peace laureate Mairead Maguire is to be deported from Israel within 48 hours after her appeal to overturn her entry refusal was rejected.
Ms Maguire was arrested at an airport in Tel Aviv on Tuesday as she arrived to attend a conference.
Israeli authorities said they refused her entry because she took part in an attempt to break the blockade of Gaza.
Ms Maguire and her lawyers are considering whether to appeal the decision.
Ms Maguire won the peace prize in 1976 for her work with the Peace People.
In June of this year, she was on board the Rachel Corrie, one of a number of ships in an aid flotilla which was refused entry to Gaza and boarded by Israeli forces.
An earlier attempt to break the blockade led to the deaths of nine Turkish citizens after an Israeli military raid on their ship.
In 1976, Ms Maguire founded Women for Peace, which later became the Community for Peace People, along with fellow Belfast woman Betty Williams.
The organisation was formed after the tragic deaths of three children.
On 10 August, Ms Maguire's sister Anne, was walking along a road in west Belfast when an out-of-control car plunged into them.
The car's driver, IRA man Danny Lennon, had been fatally wounded by a British army patrol which was chasing him.
The car plunged into the Maguires, instantly killing six-week-old baby, Andrew, who was in his pram and his eight-year-old sister, Joanne, who was on her bicycle.
Their brother John, just two-and-a-half, died the following day in hospital.
Their mother, Anne, was maimed physically and mentally - and would take her own life some years later.
It was Mairead who made a grief stricken plea for peace on television.
It struck a chord with communities across Northern Ireland and the Peace People was formed.
They marched in cities and towns such as Belfast, Enniskillen and Ballymena and held one of their most high profile rallies in Trafalgar Square in London.
Thirty-four years on, Ms Maguire continues to work for peace at home and abroad.