Margaret Beckett has been accused of having "blood on her hands" by the activist thrown out of last year's Labour conference for heckling.
Mrs Beckett said more troops were 'urgently needed'
Walter Wolfgang, now a member of Labour's national executive committee, attacked the foreign secretary's stance during the recent Lebanon crisis.
Mrs Beckett used her conference speech to say claims she had been happy for the conflict to continue were lies.
But union boss Tony Woodley said her policy had been a "political tragedy".
'Nail the lie'
Mrs Beckett praised Tony Blair's efforts to get progress on a Middle East peace deal during his recent visit to the region.
She stressed that the Israeli and Palestinian leaders were now to meet to try to secure a "two state solution", although some people would still try to obstruct those efforts.
"Let me at once nail the lie that this government actively wanted conflict to continue or was indifferent to the suffering of the people of the region," Mrs Beckett told delegates.
"From the beginning we worked intensively behind the scenes to secure the earliest possible sustainable ceasefire.
"And repeatedly G8 and EU ministers alike all called in public for a cessation of violence or hostilities and urged the combatants to respect international law and to act proportionately."
She said the UK and US had pushed for a United Nations Security Council as early as August and the delay in securing one stemmed from unease about how to make a ceasefire last.
Mrs Beckett and Mr Blair came under fire during the crisis for failing to brand the Israeli military response to Hezbollah as disproportionate or join calls for an immediate ceasefire.
Mr Wolfgang watched Mrs Beckett's speech from the side of the conference hall but said he had not been tempted to heckle this year.
He said she should have quit over the "scandal" as the UK had not done everything it could to get a ceasefire.
Lives had been needlessly lost in Lebanon because the US and UK had blocked UN Secretary General Kofi Annan's calls for an unconditional ceasefire, he argued.
"Bush and Blair and the British Government and the British Foreign Secretary have got blood on their hands and to skid over it and say we did the best, we got a ceasefire as early as it was possible to get is absolute nonsense," added Mr Wolfgang.
On the conference platform, Transport and General Workers Union chief Mr Woodley condemned the government for failing to oppose the "massacre of more than 1,000 innocent civilians".
Mr Woodley said: "We saw a country bombed back 20 years and a political tragedy for Britain.
"It was our government alone that kept backing that ludicrous Bush strategy. The world was calling for an immediate ceasefire but the inactivity of our government gave the green light to keep on bombing."
Mr Woodley said Lebanon was another conflict that had achieved nothing, like Iraq and Afghanistan.
He described Iraq as "Vietnam without the trees".
"We were not welcome and we should be out right now," said Mr Woodley.
'No failing mission'
Defence Secretary Des Browne acknowledged there were differences among Labour delegates over the Iraq war.
He said the UK was supporting an elected government in Iraq, "holding the line against vicious sectarian infighting" and trying to get the country to a point where British troops could return home without the country slipping into civil war.
"This should be something around which we can unite," he said.
Mr Browne also said the UK and its military partners must not fail in Afghanistan.
British troops have been killed during fierce fighting in the south of the country in recent weeks.
But Mr Browne insisted: "This is not a failing mission."