Mr Rantissi was killed in a helicopter missile attack in Gaza City

Rantissi was killed with two of his bodyguards in a helicopter missile attack in Gaza City.

He was recently named among Hamas leaders whose assets in the UK were frozen by Chancellor Gordon Brown.

Mr Straw's comments came ahead of Sunday's funeral of the militant Islamic movement's leader.

This assassination will confirm in the minds of the Palestinians and many in the Middle East that Israel has carte blanche

Sir Menzies Campbell, Lib Dem foreign affairs spokesman

Palestinians mourn Hamas leader

The foreign secretary said: "The British government has made it repeatedly clear that so-called targeted assassinations of this kind are unlawful, unjustified and counter-productive."

Rantissi's killing is likely to increase criticism of Prime Minister Tony Blair, who confirmed his backing for President George W Bush's support for Israeli plans to unilaterally withdraw from parts of the occupied territories.

The summit last Friday followed Mr Bush's approval for Israeli Prime Minister Ariel Sharon's proposal to leave the Gaza Strip, but retain settlements built in the West Bank in defiance of international law.

'Counter-productive'

Mr Straw was joined in his condemnation of the killing by Sir Menzies Campbell, Liberal Democrat foreign affairs spokesman.

"Coming after the Bush endorsement of the Sharon plan, this assassination, which is illegal under international law, will confirm in the minds of the Palestinians and many in the Middle East that Israel has carte blanche.

"How can anyone expect moderate Palestinians to raise their heads above the parapet in these circumstances, when opinion in their community will be so outraged?"

Senior Labour MP Donald Anderson said the killing was "clearly likely to be highly counter-productive".

The chairman of the foreign affairs committee told BBC Radio 4: "It will inflame passions and it will give another twist to this awful saga of atrocity and counter-atrocity.

"These targeted assassinations clearly do not serve any purpose in terms of finding a way through this Middle East tragedy."