Former Conservative treasurer Lord Kalms has launched a bitter attack on shadow foreign secretary William Hague over his stance on the Lebanon crisis.
Lord Kalms disagrees with Mr Hague's comments
Lord Kalms accused Mr Hague of behaving like an "ignorant armchair critic" after he said elements of Israel's response were "disproportionate".
Mr Hague's words were "downright dangerous", added the peer.
Senior Tory Oliver Letwin defended Mr Hague saying "the fact is there has been some disproportionality."
Mr Hague made his comments two weeks ago, during a Commons debate on foreign affairs - and later expanded on the theme in an article for the Sunday Telegraph.
The shadow foreign secretary told MPs the right of Israel to defend itself was "clear" and calls for an unconditional Israeli ceasefire were "futile" unless rocket attacks on it stopped and captured soldiers returned.
But, Mr Hague added, Israel's "disproportionate" response was delaying efforts to bring about a ceasefire.
"I think we can say that elements of the Israeli response are disproportionate, including attacks on Lebanese army units, the loss of civilian life and essential infrastructure and such enormous damage to the capacity of the Lebanese government, [which] does damage the Israeli cause in the long term," he told MPs.
That was still the case even if military units in the Lebanon were "callously" stationed in civilian areas, he added.
Lord Kalms said Mr Hague's comments were a product of ignorance rather than considered analysis.
"William Hague's usual good sense has deserted him," the peer said in a letter to be published in The Spectator magazine this week.
"Criticising Israel for being disproportionate without serious consideration of the alternatives merely mouths the buzzwords of the ignorant armchair critic.
"Think again, William, for whom you speak. How do you deal with the Hezbollah leader Nasrallah, who is committed to Israel's total destruction, not a single Jew to remain alive in Israel, and who rains thousands of rockets on Israel, keeping the population in shelters, devastating industry, killing and kidnapping Israeli soldiers within Israeli territory?
"A tragedy is unfolding. The outcome is life or death to the Israeli state. William, your comments are not merely unhelpful, they are downright dangerous."
Oliver Letwin - who like Lord Kalms is Jewish - said he disagreed with the peer.
"I don't think whether one happens to be Jewish or not Jewish is the issue.
"The issue here is how we can move forward to peace - at a time when both Israelis and people in the Lebanon are suffering.
"And I think that has to start by recognising that there is a real problem here of disproportionality - as well as a real problem of Israel trying to defend itself.
"And then we have to find the means of achieving a calming of the situation, so that we can move towards a ceasefire," Mr Letwin told BBC Radio One's The World at One.