UK cosmologist Prof Stephen Hawking has withdrawn from a high-profile Israeli conference, in support of an academic boycott of the country.
He initially planned to speak at the event in June but pulled out following advice from Palestinian academics.
Pro-Palestinian campaigners said the 71-year-old wrote to the organisers on 3 May saying that he had planned to criticise the Israeli government.
The conference chairman criticised the move as "improper".
Previous speakers at the Israeli Presidential Conference include former UK prime minister Tony Blair, former US president George W Bush and former US Secretary of State and Nobel Peace laureate Dr Henry Kissinger.
A statement published on Tuesday by the British Committee for the Universities of Palestine, with Prof Hawking's approval, said: "This is his independent decision to respect the boycott, based upon his knowledge of Palestine, and on the unanimous advice of his own academic contacts there."
A spokesman for the University of Cambridge - where Prof Hawking is a director of research - said the scientist had written to the Israeli president's office regarding his decision.
"We had understood previously that his decision was based purely on health grounds having been advised by doctors not to fly," the spokesman added.
The withdrawal follows representations Prof Hawking received from Palestinian academics and pro-Palestinian groups.
But he was sharply criticised by conference organisers.
"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," conference chairman Israel Maimon said in a statement.
A spokesman for the Fair Play group, which campaigns against boycotts of Israel, described the scientist's withdrawal as "bizarre".
"Prof Hawking could have joined the conference and explained his views on the conflict in the region, just as many other participants have done.
"By boycotting the conference, he has thrown away this opportunity and will help nobody."
The Israeli Ambassador in London said it was a shame Prof Hawking would not be attending.
"Rather than caving into pressure from political extremists, active participation in such events is a far more constructive way to promote progress and peace," he said.
But a source close to the boycott campaign said Prof Hawking had chosen to withdraw from the conference - rather than use it as a platform - to support Palestinian academics.
A spokeswoman for the Palestine Solidarity Campaign added: "Many will be taken aback at the extreme reaction among Israel's supporters to the news of Prof Hawking's support for the Palestinian call for boycott.
"We urge those opposed to boycotts, disinvestment and sanctions to respect freedom of speech."
Update 11 June 2013: This story has been amended following a complaint.