The Liberal Democrat leader Nick Clegg today demanded an immediate suspension of British and EU arms exports to Israel.
Mr Clegg accused Gordon Brown of "sitting on his hands" during the current conflict in Gaza and called on the Prime Minister to "unambiguously condemn" Israel's conduct of its offensive against the Hamas movement which controls the territory.
His comments, in an article in The Guardian, make Mr Clegg the first leader of a major UK party to denounce the Israeli campaign in Gaza, rather than simply calling for a ceasefire.
They come after the death of more than 30 Palestinian civilians when an Israeli shell hit a UN-run school in Gaza, where the death toll from the 12-day air and ground assault now stands at more than 600.
Mr Clegg said the world's response to the assault on Gaza was an "indictment" of the international community and accused Mr Brown of making British foreign policy "effectively subservient to Washington".
"We have an outgoing US President sanctioning Israel's military response and an aching silence from the president-elect," wrote the Lib Dem leader.
"We have a European Union encumbered by clumsy decision-making and confused messages. And at home we have a Prime Minister talking like an accountant about aid earmarked for Gaza without once saying anything meaningful about the conflict's origins."
While Israel has every right to defend itself, its use of overwhelming force in a bid to stop Hamas rocket attacks is "self-defeating" because it is radicalising Palestinian and Arab opinion, said Mr Clegg.
"Brown must stop sitting on his hands," he said. "He must condemn unambiguously Israel's tactics, just as he has rightly condemned Hamas's rocket attacks.
"Then he must lead the EU into using its economic and diplomatic leverage in the region to broker peace."
Britain's arms export approvals to Israel have risen from £6 million in 2007 to £20 million in the first three months of 2008 alone, said Mr Clegg.
And he added: "Brown must... halt Britain's arms exports to Israel, and persuade our EU counterparts to do the same. I want an immediate suspension of all arms exports from the EU, but if that cannot be secured, Brown must act unilaterally."