The SS Free Gaza and the SS Liberty, which are sailing from Crete to Cyprus and then on to Gaza after being delayed by a storm, will carry about 40 protesters campaigning against Israel's economic sanctions on the Palestinian territory, including an 84-year-old Holocaust survivor and Lauren Booth, sister-in-law of Tony Blair, who is the international special envoy to the Middle East.
The boats, which will also carry a cargo of balloons, musical instruments and thousands of hearing aids, are expected to approach Gaza early next week.
The territory's land borders have been closed to all but essential humanitarian aid for more than a year, plunging it into deep poverty, and the tiny strip of land is rarely visited by foreign dignitaries. An attempt by Mr Blair last month was aborted at the last minute due to security concerns.
Israeli officials are thought to see the protesters' efforts as a dangerous precedent. The foreign ministry sent a letter advising organisers that Gaza coastal waters are subject to a no-go warning from the Israeli navy and that any attempt to approach would be interpreted as assistance to a terrorist regime.
They have offered help in delivering humanitarian aid via land borders instead.
"From my point of view this is some kind of pirate ship," said Shlomo Dror, a defence ministry spokesman.
"You can demonstrate, that's OK with us. But you are not allowed to break international law."
Gaza has been subject to heavy economic sanctions since the Islamist militant group Hamas violently wrested control there in June 2007. Hamas has refused international demands to recognise Israel and renounce violence.
Though shipments have increased under the terms of a ceasefire with Israel, shortages of many basic goods continue and a United Nations report last month found Gaza now has the world's highest unemployment rate, at 45 per cent.
Organisers from the Free Gaza movement, who raised £150,000 for their mission to challenge the blockade, maintain Israel has no right to stop the boats. But they acknowledge they are likely to be intercepted by the Israeli navy and detained, and have already contacted consular officials and lawyers in Israel.
Defence officials say unauthorised vessels would normally be escorted to official seaports at Ashdod or Haifa.
"We hope that the Israeli government will have some wisdom. To drag us in and arrest us and say somehow we are a danger is absurd," said, Greta Berlin, one of the organisers who is in Cyprus, where the boats are expected to dock before continuing to Gaza over the weekend.
"Of course we're going to make a point. Gaza has the right to have its own seashore ... We intend to break the siege."
Under the 1993 Oslo Accords, Israel retained military control of Gaza's territorial waters but agreed to allow fishing boats within 20 nautical miles. Today fishing is permitted only within about three nautical miles and boats that venture further risk Israeli fire.
The highly controversial journey, announced earlier this summer, has already been delayed several times because of bad weather. Participants, including Ms Booth, say they have received death threats for their efforts.
"An anonymous young man called my home in France as my daughters played hide-and-seek in the garden," Ms Booth wrote on her blog.
"This stranger spoke to my husband, warning him that 'your wife is in great danger. These ships will be blown up.' My husband asked how it was this person had obtained our private home number.
"No response was forthcoming but the illicit threats carried on."
