Former cabinet minister Clare Short has been criticised for hosting a parliamentary question-and-answer session with Khaled Mashal, head of Hamas’s political bureau.
Mr Mashal was due to appear by satellite link at Wednesday evening’s “A Talk with Hamas” event.
The hardliner has repeatedly ruled out recognition of Israel and advocated suicide attacks. He was the target of a botched Mossad assassination attempt in Jordan in 1997.
Last month Ms Short led a delegation of MPs to meet Mr Mashal in Damascus, where he lives in exile.
Conservatives wrote to the Speaker of the House urging him to reconsider allowing parliamentary facilities to be used for the meeting.
Shadow Foreign Affairs Minister David Lidington said: “We are seriously concerned. Hamas remains a proscribed terrorist organisation under UK law. It would be inconceivable that Mr Mashal would be permitted to travel in person to address MPs.”
James Arbuthnot MP, parliamentary chair of the Conservative Friends of Israel, said: “This makes a mockery of parliamentary institutions as well as painting parliament as soft on terrorism.”