The ships, carrying about 10,000 tonnes of aid and up to 700 passengers on six vessels, were warned not to pass an Israeli sea blockade.

Among those travelling on the aid ships were eight Irish citiizens.

Michael Martin, Irish foreign affairs minister, said: "I am gravely concerned at the reports emerging of the storming of a Turkish ship by Israeli commandos.

"The reports of up to 15 people killed and 50 injured, if confirmed, would constitute a totally unacceptable response by the Israeli military to what was a humanitarian mission attempting to deliver much needed supplies to the people of Gaza."

The Stop The War Coalition (STWC) said the action "should see Israel condemned under international law".

An STWC spokesman said: "The 600 protesters were on a peaceful mission and unarmed. The flotilla was an attempt to bring aid to the blockaded population of Gaza. It was supported by many organisations and individuals internationally.

"The decision by Israel to attack the flotilla with such loss of human life shows its arrogant and deadly intent in opposing any aid to the Palestinians."

The Turkish Mavi Marmara ship, from the pro-Islamic aid group IHH, and Free Gaza's Challenger 1 are understood to have been boarded by Israeli Navy ships in international waters, 80 miles off the coast of Gaza.

A spokesman for the Israeli embassy in London said: "We have no doubt regarding the real intention of the flotilla, it's not about humanitarian aid. You can see clearly from the footage that when they boarded they were attacked with knives and sharp metal objects and left with not much option but to respond. There was no intention whatsoever to use any of the weapons soldiers naturally carry. As soon as the soldiers boarded they were attacked by knives and life-threatening objects."