In a sign of worsening of relations with plant owner Ineos, Unite will debate whether to link backing for Labour at Westminster and Holyrood with support for nationalising the company's operations without compensation.
Unite's attempt at influencing Labour's Westminster selection contest in Falkirk, in which the union signed up more than 100 new members in a bid to help their favoured candidate, has big political ramifications.
The sign-up, which was linked to the Ineos plant at Grangemouth, led to the party rethinking its links with all its trade union affiliates. It also spiralled into a major industrial dispute.
It has now emerged Unite will reopen the Ineos issue at its UK policy conference in June. A copy of the preliminary agenda contains a motion signed by two Scottish branches.
It condemns the "threat by Ineos's Ratcliffe [Jim, Ineos chairman] to shut down operations in Grangemouth" and said it was "unacceptable" for "one individual to be able to wield such power".
The motion also commits Unite to "campaign for the nationalisation without compensation and under workers' control of all Ineos assets in the United Kingdom".
It called for this demand to be a "major focus" of the union's campaigning in the run-up to the next Westminster and Holyrood elections, including "withholding support from any candidate who does not support that demand".
Unite is the biggest union donor to Labour, but the party has not backed calls to seize any of Ineos's assets.
If passed, the motion could result in only a handful of left-wing Labour candidates receiving funds from Unite and starve Ed Miliband of resources.
Ineos and Scottish Labour both declined to comment.