Israel last night warned an attempt by peace activists to sail two wooden boats to the Gaza Strip was a "provocation" and said it would prevent them reaching their destination.

A group of 46 activists set sail yesterday morning from Cyprus and were hoping to reach Gaza later today to challenge the economic blockade Israel has imposed on the strip and to deliver a cargo of 200 hearing aids for a deaf school and 5,000 balloons. Among those on board is a Catholic nun, aged 81, the British journalist Yvonne Ridley and Lauren Booth, sister-in-law of Tony Blair.

"I've been nervous, but today I'm excited," said Booth, 41, shortly before the boats sailed. "It's not about our fear, it's about the people waiting in Gaza, you can't think about anything else."

Another passenger, Huwaida Arraf, a Palestinian with Israeli and US citizenship, said: "The siege that the Israelis have imposed on Gaza is not only illegal in terms of international law, it is also immoral. Global institutions and the governments of the world know what is happening and are not doing anything about it."

Israel has already warned those on board not to undertake the journey and last night Aviv Shiron, spokesman for Israel's foreign ministry, said the voyage was a "provocation" and that "all options" were under consideration to prevent the boats reaching Gaza. An Israeli admiral has issued an apparent warning to all shipping to stay clear of waters near Gaza because of an unspecified "security situation."

It appears most likely that there will be a standoff with the Israeli navy later today, and that the activists will be arrested rather than being allowed into Gaza. Although Israel withdrew its soldiers and settlers from Gaza in 2005, it still controls Gaza's air space and sea space, as well as nearly all the border crossings. Until a recent ceasefire with Hamas the Israeli military was mounting regular incursions into the strip, saying it wanted to stop rocket fire into southern Israel. It has imposed a tight economic blockade of Gaza, saying it wants to weaken Hamas, the Islamist group which won Palestinian elections more than two years ago.

In a statement issued as they departed yesterday, the activists said they would lodge a legal protest against any attempt by the Israelis to arrest them.

"If Israel chooses to forcibly stop and search our ships, we will not forcibly resist," they said. "If we are arrested and brought to Israel, we will protest and prosecute our kidnapping in the appropriate forums ... It is our purpose to show the power that ordinary citizens have when they organise to stand against injustice."