Israel has already blocked two attempts by activists to deliver aid by ship to Gaza, in June and July.
While the Israeli government has characterised the flotilla as a “selfie yacht”, Thunberg has pushed back against that criticism, telling the BBC on Sunday: “I don’t think anyone would risk their life for a publicity stunt.”
International aid agencies have been attempting to get food and medicine into the Palestinian territory but note Israel is restricting the flow of supplies.
Israel claims it is attempting to stop those supplies falling into the hands of Hamas. It and the US have backed an alternative food distribution system, the Gaza Humanitarian Foundation (GHF) which the UN refuses to co-operate with, describing its set-up as unethical.
A UN-backed group confirmed last month that there was famine in Gaza and the UN’s humanitarian chief said it was the direct result of Israel’s “systematic obstruction” of aid entering the territory.
Israeli Prime Minister Benjamin Netanyahu has called this an “outright lie”.
French Foreign Minister Jean-Noel Barrot said in a statement that France had ensured “that any possible boarding operation would take place under the best possible security conditions”.
Italy’s foreign minister said he had been reassured by Israel that its armed forces would not use violence against the 500 people aboard, including French and Italian politicians.
Antonio Tajani said: “The boarding was planned, we are talking about it… with [Israeli Foreign Minister Gideon] Saar so that there would be no violent actions on the part of the Tel Aviv armed forces, and this has been assured to me.”
Simon Harris, Ireland’s tánaiste (deputy prime minister), said his country “expects international law to be upheld and all those on board the flotilla to be treated in strict accordance with it”.
In Gaza, Israel is stepping up its assault on Gaza City as Hamas weighs its response to a new US plan to end the war. Arab and Turkish mediators are understood to be pressing Hamas for a positive response, but a senior Hamas figure has said the armed group is likely to reject it.
Israel’s Defence Minister Israel Katz issued a final warning to the hundreds of thousands of Palestinians in the city to evacuate southwards, saying those who remained during the offensive against Hamas would be “terrorists and supporters of terror”.
The International Committee of the Red Cross stated that “under international humanitarian law, civilians must be protected whether they stay or leave Gaza City”.