British and Irish people are among a group of 21 human rights activists "kidnapped" by the Israeli authorities, campaigners said.
The group were on board an aid boat carrying humanitarian supplies near the Gaza coast when they were seized in international waters, according to the Free Gaza Movement.
The Spirit of Humanity was on its way to deliver medical supplies to Gaza when it was allegedly stopped by Israeli warships on Tuesday morning with threats of gunfire.
The people on board who were "forcibly" taken to Israel included eight from Britain and Ireland as well as fellow activists from the US, Denmark and the Middle East.
The Foreign Office said it was aware of the reports and was trying to clarify the facts.
A spokeswoman said on Tuesday night: "We would be concerned if the stories of the Israeli Navy boarding the boat in international waters were true.
"We have made it clear to Israel that we are very concerned for the safety of British nationals."
Nobel laureate Mairead Maguire and former US Congresswoman Cynthia McKinney were among the group on the ship, taking aid to Gaza in the aftermath of its bombardment by Israel.
Their fellow activists included Ishmahil Blagrove, a Jamaican-born journalist based in the UK; Derek Graham, an Irish electrician: the ship's captain Denis Healey from the UK; British journalist Fathi Jaouadi; Alex Harrison, a British woman; Theresa McDermott from Scotland; Adnan Mormech from the UK; and Ms Maguire.
