The Trades Union Congress is expected to call on its members at its annual conference to strengthen opposition to Israel by extending boycotts and encouraging disinvestment.
A motion on Israel and the Palestinians, backed by the GMB, the Fire Brigades' Union, the Public and Commercial Services Union and the Transport Salaried Staffs' Association, is expected to propose a full boycott of Israeli goods.
The four-day conference begins in Manchester on September 13.
Eric Lee of TULIP (Trade Unions Linking Israel and Palestine) warned that Israel supporters should "not expect good news" from the congress.
Trade Union Friends of Israel (TUFI) will host a fringe meeting discussing the role unions can play in the peace process. TUC general secretary Brendan Barber has been invited to speak at the event.
Last year, in the wake of Operation Cast Lead in Gaza, the TUC agreed to support a boycott of goods from Israeli settlements. The move was described at the time as a "profound shift in the culture of the British trade union movement where Israel is concerned".
Relations with the Histadrut (Israeli TUC) are likely to be strained further by this year's proposals. The main motion on the conflict will condemn "Israeli blockades of the Palestinian territories, particularly the Gaza Strip".
Specific condemnation will be reserved for "the actions of the Israeli military" during May's Gaza flotilla incident.
The motion will also criticise the "effective annexation of massive swathes of land by Israel" and call on the government and EU to take "much stronger political steps" against Israel.
The congress is expected to also push for further disinvestment from companies profiting from "the occupation and the construction of the wall".
Mr Lee said: "Former union leaders would have got up and stopped these things but they are not around any more. Brendan Barber is not a block on such moves. It's not going to be good news, certainly."
A report by the TUC's general council will be presented at the conference and will focus on the federation's work in the past year.
On the Middle East it will recall that the congress pressed the government to stop arms sales and suspend trade agreements with Israel following Cast Lead, but record that the government's response fell "well short" of the total embargo demanded.
The summary will also record that the congress supported guidance from the Department for Environment, Food and Rural Affairs (Defra) on labelling of goods from Israeli settlements in the West Bank.
The TUC worked with the PSC to create a "ban settlement goods" postcard targeting MPs, and a "would you buy stolen goods" card for shoppers. Mr Lee added: "The TUC does not tend to get more moderate about these things. The full backing of the PSC is a major move. It was a terrible shift.
"Several people last year thought it was almost a positive result for Israel, because the motions stopped short of a full boycott. I hope this year people are honest about what happens. If we are defeated, we should say so."