Liberal Democrat leader Sir Menzies Campbell has called for an end to the export of British arms to Israel.
Sir Menzies has led calls for an unconditional ceasefire
Sir Menzies says the exports break the government's rules because the UN has called Israeli attacks "collective punishment" of Lebanese people.
Tony Blair declined to comment on Sir Menzies's plea earlier on Monday.
The prime minister told a press conference in Downing Street he was working with the UK's allies on a ceasefire plan.
He said what was happening in Lebanon was a "catastrophe" but innocent Israeli civilians had been killed too.
He rejected claims the UK and US did not want an immediate ceasefire - and that they had taken sides with Israel.
But he said a plan had to be drawn up that would satisfy the demands of both sides.
Sir Menzies, who is leader of the UK's third biggest party, has led calls for an unconditional and immediate ceasefire.
In a letter to Mr Blair, he said: "In light of disproportionate military action by Israel in Lebanon and Gaza the UK government must suspend any further arms exports to Israel.
"Over the last year and a half the UK government has licensed the export of arms to Israel worth over £25m, including components for naval light guns, military utility helicopters, aircraft radars and electronic warfare equipment.
"The government is right to ensure there are no arms transfers, either direct or indirect, from the UK to Syria, Iran or illegal armed groups such as the military wing of Hezbollah.
"Kofi Annan has said attacks in Lebanon are inflicting collective punishment on the Lebanese people and the UN Emergency Relief Co-ordinator has described attacks on Beirut as a 'violation of humanitarian law'.
"The government must now comply with its own arms export rules and institute an immediate suspension of all UK arms exports to Israel."
Mr Annan, the UN secretary general, had said the attacks were "disproportionate", Sir Menzies pointed out.
And a ban on arms exports "would be an indication that the United Kingdom government recognises the strength of the judgment of the Secretary General which up to now at least, with the possible exception of Kim Howells, the government has declined to do."
Speaking on BBC Radio 4's PM programme, Sir Menzies said Israel's actions "may well be" a war crime but that would have to be the subject of an investigation.
It would be "very difficult" to find British forces to join an international force due to commitments in Iraq and Afghanistan, he said.