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Greer Fay Cashman contributed to this report.

LONDON – After much back and forth on Wednesday, Prof. Stephen Hawking retracted his statement that he pulled out of the fifth President’s Conference in Jerusalem because of ill health and clarified that it was because he supports an academic boycott of Israel.Earlier on Wednesday, the physicist’s spokesman at the University of Cambridge had denied that support for a boycott was the catalyst and said that he withdrew on medical grounds.“Professor Hawking has decided to cancel his planned visit to Israel on the advice of doctors,” the spokesman said.A letter sent by Hawking to the organizers of the June 18- 20 conference suggested, however, that pressure from Palestinian academics made him withdraw.“I have received a number of emails from Palestinian academics,” Hawking said in the letter. “They are unanimous that I should respect the boycott. In view of this I must withdraw from the conference.”When this was put to his spokesman, he told The Jerusalem Post that he was unaware of this line in the letter and reaffirmed that medical reasons was the sole reason for canceling.Later on Wednesday, the University of Cambridge withdrew its original statement and confirmed that Hawking withdrew his participation based on advice from Palestinian academics that he should respect the boycott.“We had understood previously that his decision was based purely on health grounds, having been advised by doctors not to fly,” the university spokesman said.Presidential Conference chairman Israel Maimon responded to Hawking’s announcement on Wednesday saying that his decision was unjustifiable and wrong.“The academic boycott against Israel is in our view outrageous and improper, certainly for someone for whom the spirit of liberty lies at the basis of his human and academic mission. Israel is a democracy in which all individuals are free to express their opinions, whatever they may be. The imposition of a boycott is incompatible with open, democratic dialogue.”Nitsana Darshan-Leitner, director of Shurat HaDin – Israel Law Center, called Hawking’s boycott hypocritical.“His whole computer-based communication system runs on a chip designed by Israel’s Intel team. I suggest that if he truly wants to pull out of Israel, he should also pull out his Intel Core i7 from his tablet.”She suggested that he should also consult his academic contacts and the Intel engineers in Israel before deciding to boycott.“He seems to have no understanding of this world.”Since 1997, Hawking’s computer- based system has been sponsored and provided by Intel.This year’s Presidential Conference will also be a celebration of the 90th birthday of President Shimon Peres.Organizers are expecting a record attendance of 5,000 participants, including some of the world’s greatest brains, leaders of the largest global technological companies, internationally renowned academics, Nobel Prize laureates, celebrated artists and past and present heads of state. Former US president Bill Clinton is to be awarded with the Presidential Medal of Distinction at the conference.Other statesmen who are set to join Peres in celebrating his ninth decade include former president of the Soviet Union Mikhail Gorbachev, former British prime minister Tony Blair, who attended all four previous Presidential Conferences, and Prince Albert of Monaco.Legendary singer Barbra Streisand will divide her time in Israel between conference participation in Jerusalem and her sold-out concerts in Tel Aviv.Although Hawking’s initially scheduled participation in the conference had captured headlines in Israel, it is doubtful that there would have been any public response to his cancellation if other media outlets had not picked up on a report in The Guardian.Hawking, 71, has visited Israel four times, most recently in 2006, when he lectured at Israeli and Palestinian universities.In an interview with Al Jazeera during Operation Cast Lead in 2009, Hawking condemned Israel, saying its response to Gazan rocket fire was out of proportion and compared it to apartheid South Africa.