The FBU condemns Israel for its offensive in Gaza in January
UK union leaders have been forced to suspend a debate on calls for a boycott of Israeli goods after heated exchanges between senior officials.
The issue has divided the movement and led to acrimonious scenes at the annual TUC congress in Liverpool.
Delegates were due to vote on the proposed boycott motion, put forward by the Fire Brigades Union, on Thursday.
The GMB has called the move "divisive" but the motion has the support of the powerful Unite and Unison unions.
The BBC's employment correspondent Martin Shankleman said the issue had led to an angry confrontation between TUC General Secretary Brendan Barber and Unite leader Derek Simpson.
As well as a boycott, the motion calls for a ban on imported goods from "illegal Israeli settlements in the occupied territories", an end to arms sales to Israel and the suspension of official links between British and Israeli trade unions.
Senior GMB official Paul Kenny said the proposal was "incredibly divisive", describing it as "way beyond the logic of where we should be".
But the Fire Brigade Union's Mick Shaw said there was strong support for the move among Palestinian people and it would put pressure on Israel over its military offensive in Gaza in January.
Palestinians and human rights groups say more than 1,400 Gazans were killed during the three-week operation. Israel puts the figure at 1,166.
Israel's deputy ambassador in London said such a move would not help prospects of peace in the region.
"It will only harm workers in our area," Talya Lador-Fresher told BBC Radio 4's Today programme.
"It will harm both Palestinian workers, 50,000 of which are working in Israel and the West Bank, and it will also harm Israeli workers. It may even backfire and harm British workers in this country because boycotts have a tendency to work both ways."
The TUC General Council decided to support the motion on Tuesday and put it to a vote of delegates.
However, our correspondent said the controversy around the subject may mean that Thursday's vote will be fudged.