Three Britons were among seven aid workers killed in air strikes in Gaza carried out by Israel’s army on Monday.
It has led the Lib Dems, SNP and some Labour and Conservative MPs to urge the UK to reassess how it supports Israel.
The Foreign Office said on Thursday it kept advice on Israel’s adherence to the law under review, adding that this was confidential.
Writing in his Daily Mail column, Mr Johnson said it would be “insane” for the UK to end military support, which he said would be “willing the military defeat of Israel and the victory of Hamas”.
Mr Johnson, a former foreign secretary, wrote: “Remember that in order to win this conflict, Hamas only has to survive. All they need at the end is to hang on, rebuild, and go again.
“That’s victory for Hamas; and that is what these legal experts seem to be asking for.”
Three Britons were among the World Central Kitchen (WCK) workers who died in the strikes: John Chapman, 57, James “Jim” Henderson, 33, and James Kirby, 47.
WCK founder Jose Andres claimed the Israeli military knew of his aid workers’ movements and targeted them “systematically, car by car”.
Israeli officials reacted by sacking two officers over the strikes, which were described as a “grave mistake stemming from a serious failure”.
Mr Johnson said it was “shattering” to see the deaths, but Israel was sending warnings of their attacks and “trying to use precision munitions”.
Foreign Secretary Lord David Cameron previously said the UK would carefully review findings of the initial Israel Defense Forces (IDF) report into the incident, which was released on Friday.
In addition to calls from MPs, a letter signed by more than 600 lawyers, including former Supreme Court justices, has also called for all major parties to end UK arms sales to Israel.
Much of the Gaza Strip has been devastated during the IDF military operations which began after Hamas gunmen attacked southern Israel on 7 October, killing about 1,200 people and seizing 253 hostages.
More than 33,091 people have been killed in Gaza since then, the Hamas-run health ministry says.
On Thursday a UK Foreign Office spokesman said: “We keep advice on Israel’s adherence to international humanitarian law under review and ministers act in accordance with that advice, for example when considering export licences.
“The content of the government’s advice is confidential.”