Israel says the expulsion of activists - including over 40 Britons - detained during the storming of an aid flotilla heading for the Gaza Strip will begin "immediately".
All 679 detainees taken off the ships during the military-led action which saw nine civilians killed are expected to be deported within 48 hours, a statement on the Israeli prime minister's office website said.
Earlier, Foreign Secretary William Hague said 31 British nationals and a further 11 with dual nationality were known to have been detained after the seizure of the vessels as they attempted to breach the Israeli blockade of the territory.
Following a meeting of Israel's security and diplomatic cabinet, the statement said: "It was agreed that the detainees would be expelled immediately - according to the procedures set by law.
"The interior minister announced that the expulsion will begin this evening, and the assessment is that it can be completed in approximately 48 hours."
The Foreign Office said it is seeking "urgent clarification" of the situation.
Mr Hague had welcomed a statement by the United Nations Security Council calling for a "prompt, impartial, credible and transparent" investigation into the seizure of the flotilla by Israeli commandos early yesterday.
"We look to Israel to co-operate to implement that resolution with a transparent and thorough investigation," he said.