Roey Gilad, the Israeli Embassy spokesman in London, had been invited to the college, which is part of London University, to take part in a debate organized by the student union's Jewish Society.
However, the union voted to ban his appearance and the society was sent an email explaining that any speaker from the Israeli embassy would go "against union policy."
'A political institution committed to freedom of speech'
College authorities intervened and insisted that the student union reverse its decision, the Guardian reported.
"It contradicts our commitment to freedom of speech," college director Colin Bundy said. "I think this comes with the turf of being a very political institution."
The union said they would accept the university's decision and allow Gilad to speak at the debate, but would not actively support the society's event.
Union co-president in-charge of societies Kavita Meelu said the union had formed a policy last November stating that the union's basic belief is the elimination of "foreign occupation, apartheid, (and) Zionism," as a prerequisite for peace.
Gilad said he believes this violates freedom of speech.
"It prevents someone presenting his agenda, and someone who represents a legitimate state," he said. "I didn't expect this in the U.K., let alone at a university."