On Tuesday, Mideast envoy Tony Blair called off what would have been the first visit of a top Western diplomat to Hamas-ruled Gaza, after Israel's Shin Bet security service received "pinpointed and imminent" intelligence that there was going to be an assassination attempt on his life.

The Shin Bet security service said it had received "information that Palestinians were planning to attack Blair in Gaza, so the relevant services alerted him to the fact."

They say the information about the attack was "detailed and credible." Militants planned to attack Blair's convey with explosives while he was traveling in Gaza.

Blair's visit Tuesday was to have included a tour of a Gaza waste-water project and meetings with traders and U.N. officials, but not with leaders of Hamas, the Islamic militant group that seized Gaza by force more than a year ago.

Still, Hamas had made security arrangements for Blair, setting up checkpoints in areas he was expected to tour, banning cars from using roads, and lining streets with black-clad policemen carrying AK-47s.

Since the Hamas takeover, Gaza has been virtually sealed off from the world by Israel and Egypt, a policy that has received tacit international backing.

Blair has said in recent weeks that a new policy toward Gaza needs to be developed, pointing to the growing suffering of Gaza's people, but has not offered a plan. The options are limited because much of the international community considers Hamas a terrorist group and has shunned its government.

Blair's spokesman, Matthew Doyle, said the envoy called off the visit "due to a specific security threat which would have made it irresponsible to proceed, not just for those visiting but also the local community."

"He looks forward to being able to go to Gaza again in the future and will of course in the meantime continue to work to improve the conditions for the people there," Doyle added.

Hamas Government Spokesman Taher Nunu denied there were any security threats against Blair. "Gaza is still open for all visitors, to break the siege and see the extent of suffering here," he said.

Although the once lawless Gaza has been mostly pacified under Hamas' stern rule, there are still shadowy extremist Muslim groups in the territory. On an Islamist forum popular with Gaza residents, some users slammed Blair's expected visit, but there were no direct threats of violence against him. Those comments were later removed from the Web site.

A key stop on Blair's trip would have been a northern Gaza waste water project being built with international funds. The Mideast envoy had not been expected to meet officials from Hamas, which is committed to Israel's destruction and is considered a terrorist group by the U.S., EU and Israel.

"We are very disappointed," said John Ging, Gaza director of the U.N. agency in charge of aiding Palestinian refugees.

Blair, a former British prime minister, is trying to revive the struggling Palestinian economy to lay the groundwork for a future independent Palestinian state.

He represents the Mideast Quartet — the U.S., Russia, the European Union and the United Nations — which is trying to push the Palestinians and Israel toward a peace agreement.

Blair also canceled a Tuesday visit to Sderot, a town in southern Israel which is the frequent target of rockets fired by Palestinian militants in Gaza.

In October of 2003, U.S. officials were in Gaza interviewing potential Fulbright Scholars when their convey was attacked with explosives. Three security guards riding in an armored vehicle were killed. Since that incident no American officials are supposed to go into Gaza.

The Associated Press contributed to this report.