The FBU condemns Israel for its offensive in Gaza in January
Delegates at the TUC Congress will vote on a call to boycott Israeli goods and ban the importation of products from "illegal Israeli settlements".
The motion has been put forward by the Fire Brigades Union (FBU) and it is supported by Unite and Unison.
Union leaders were involved in a heated debate on Tuesday before deciding to back the move.
Israel's deputy ambassador in London said it would help the "radicals burn all hopes for reconciliation".
Talya Lador-Fresher told BBC Radio 4's Today programme it was a "pity" the motion came from the FBU, whose "job is to take out fire but in this case, setting fire on".
"Boycotts will not bring the peace process forward. It will only harm workers in our area. 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," she said.
"It may even backfire and harm British workers in this country because boycotts have a tendency to work both ways."
She added it was a pity "extremists" in the TUC had "hijacked the agenda".
The FBU will call for an end to arms trading with Israel, a ban on importing goods from "illegal Israeli settlements in the occupied territories" and promotion of a consumer-led boycott.
It wants members of the TUC to boycott Israeli goods, especially agricultural products produced in the settlements.
The FBU is taking a stand against Israel's offensive in Gaza in January. Palestinians and human rights groups say more than 1,400 Gazans were killed during the three-week operation, but Israel puts the figure at 1,166.
The TUC General Council decided to support the move after a meeting which is believed to have ended in acrimony.
The leaders were holding their usual meeting to discuss the General Council's position on motions ahead of Wednesday's TUC Congress in Liverpool.