Members are being urged to consider the implications of education links
Academics have agreed to reconsider their ties with Israeli universities, prompting accusations from Jewish groups that they are out of touch.
The University and College Union told its congress it had ruled out a boycott of Israeli institutions over the Palestinian conflict.
Instead, members voted to "consider the moral and political implications of education links with institutions".
But Jewish leaders said the motion could pave the way for a boycott.
About 30 of the 250 delegates at the annual University and College Union (UCU) congress in Manchester opposed the motion brought by Tom Hickey, a lecturer in philosophy at Brighton University.
Last year, a similarly-worded motion was passed and met with anger by Jewish groups in Israel and the UK. At that time, legal advisors warned union leaders a boycott would be illegal.
A UCU delegation visited the Palestinian territories earlier this year on a fact-finding mission as a result of last year's vote.
Mr Hickey told delegates this latest motion had emerged from the decision "not to boycott but to discuss the issue and organise a tour and that is what we did".
"We refuse to be intimidated...We will protect the union from legal threat but we will not be silenced on this issue," he said.
UCU president Linda Newman said she would not support a boycott but supported the motion.
But Labour MP John Spellar said the motion ran "contrary to the views of ordinary members and against principles of academic freedom".
The move has also reignited anger among Jewish groups.
Jeremy Newark, head of the Stop the Boycott campaign, said: "UCU has again demonstrated how out of touch it is with the vast majority of its membership and the wider academic community.
"This motion does nothing to help the Palestinians."
The Stop the Boycott campaign is supported by Jewish groups, including the Board of Deputies, the Union of Jewish Students and the Trade Union Friends of Israel.