The chief executive of the Britain Israel Communications and Research Centre is to step down after nearly six years in the role.
Lorna Fitzsimons, who served as a Labour MP for Rochdale for eight years before joining Bicom, is leaving to pursue other opportunities.
Ms Fitzsimons, who was president of the National Union of Students between 1992 and 1994, took the job at Bicom in 2006 after Danny Shek stepped down.
Under her supervision, Bicom held the We Believe in Israel conference last year, attracting 1,500 people and nearly 100 guest speakers.
Bicom was set up in 2001 to counter anti-Israel propaganda. When Ms Fitzsimons, 44, was appointed its chief executive, she said that "Israel was at the forefront of a struggle that has a direct impact on us here in the UK.
Ms Fitzsimons said working for Bicom had been "the most amazing experience and privilege".
"To be able to work on an issue that you also believe in and care about is a luxury not many people have. My belief in what we do and its importance has not diminished and will not in the future.
"Now more than ever Israel needs friends and I can assure you I will never stint in my efforts to promote and defend her.
"I am leaving the organisation in good hands with Dermot Kehoe leading the team going forward." Mr kehoe will lead Bicom until a new chief executive is appointed.
Bicom board chairman Poju Zabludowicz said the board had reluctantly accepted her resignation.
"Under Lorna's leadership Bicom has reached exemplary levels. She leaves a huge mark on the organisation, the pro-Israel community and Israel advocacy."