The boats of the second Gaza flotilla are expected finally to set sail this weekend from the Greek coast.

The flotilla was initially planned to sail on Tuesday but a number of delays, including a mysteriously broken propeller shaft and anonymous complaints to the Greek Coast Guard, have delayed embarkation.

Several boats have already set sail from Corsica and Spain while the majority of the flotilla is yet to leave Greece. They plan to rendezvous at sea south of Cyprus from where they are to sail towards Gaza.

Israeli Navy craft and Air Force helicopters are on alert to intercept the flotilla at sea, as the organisers are expected to ignore calls not to enter the blockaded area and to unload the supplies meant for Gaza at Ashdod in Israel or El-Arish in Egypt.

Lessons have been learned from last year's flotilla when nine Turkish activists were killed in the violence directed against Israeli naval commandos who tried to board one of the boats, the Mavi Marmara.

This time, Israel is keeping a close eye on the flotilla and especially the intentions of its passengers. The naval commando unit, Flotilla 13, has rehearsed a wide range of scenarios, from a peaceful takeover of the boats to countering violent opposition.

A senior military source said this week: "We don't want to give them any PR prizes and therefore we will try every possible way to stop them from arriving in Gaza without actually having to board the boats. If we have no choice, we will board but we want to do everything possible to prevent casualties."

Following a meeting of the Israeli Cabinet on Monday, Prime Minister Benjamin Netanyahu instructed the IDF to block the flotilla, if necessary by force.

Meanwhile, Israeli intelligence officers claimed on Monday that some of the flotilla's passengers were "planning to kill Israeli soldiers" and that they had prepared chemicals to pour on the soldiers. Dror Feiler, an Israeli-Danish activist who is one of the flotilla's organisers, denied this, saying: "Israel is trying to justify violence in advance. There is no one on the flotilla who plans to use violence. If there were, we wouldn't allow them on board."

Israel has, however, backed down on one issue. After sending a letter to the Foreign Press Association on Sunday, saying that journalists who report from the flotilla could be barred from visiting Israel for 10 years - just as the activists will be if they are arrested - the Prime Minister's Office retracted the threat 24 hours later.

Mr Netanyahu was apparently not consulted over the letter and, following the outraged response of international journalists based in Israel, he ordered a special procedure to be formulated for reporters on the flotilla.

In addition to journalists, on the boats will be a wide selection of politicians, writers and activists. Perhaps the most famous will be American poet and Pulitzer Prize-winner Alice Walker, who has been writing extensively on her reasons for joining the flotilla; and Swedish crime writer Henning Mankell, who was on the previous flotilla.

Other Americans include 87-year-old Holocaust refugee Hedy Epstein. There will be a number of parliamentarians from countries as diverse as Ireland and Malaysia, and several Britons.