Britain's Foreign Secretary David Lammy said it was "unacceptable" that Israel had detained two UK lawmakers and denied them entry.
Yuan Yang and Abtisam Mohamed, from the governing Labour Party, flew from London to Israel but were blocked from entering the country and deported, British media reported.
"It is unacceptable, counterproductive, and deeply concerning that two British MPs on a parliamentary delegation to Israel have been detained and refused entry by the Israeli authorities," Lammy said in a statement.
"I have made clear to my counterparts in the Israeli government that this is no way to treat British parliamentarians, and we have been in contact with both MPs tonight to offer our support.
"The UK government's focus remains securing a return to the ceasefire and negotiations to stop the bloodshed, free the hostages and end the conflict in Gaza."
In a joint statement, the two MPs said they were "astounded at the unprecedented step" taken by Israel.
But Israel said their claim to be part of an official parliamentary delegation was "found to be false, as no official body in Israel was aware of such a delegation's visit".
The incident sparked a domestic row between Lammy and Conservative leader of the opposition Kemi Badenoch, who told Sky News on Sunday that she was "very concerned about a lot of the rhetoric" from Labour MPs on Israel and that she was "not surprised" the pair had been detained.
Lammy responded by writing on X: "It's disgraceful you are cheerleading another country for detaining and deporting two British MPs."
Since renewed military operations last month ended a short-lived truce in its war with Hamas, Israel has pushed to seize territory in the Gaza Strip in what it said was a strategy to force militants to free hostages still in captivity.
The health ministry in Hamas-run Gaza said 1,249 people have been killed since Israel resumed intense bombing last month, bringing the overall death toll since the war began to 50,609.
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