Abtisam Mohamed (L) and Yuan Yang (R)
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."
The Palestinian health ministry 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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