The High Court is to rule on allegations against the Government of unlawfully seeking to restrict "ethical" boycotts on Israel, which has long been accused of violating Palestinian human rights.
The case, brought by the Palestine Solidarity Campaign (PSC), concerns guidance affecting the Local Government Pension Scheme (LPGS) and how its funds are - and are not - invested.
The guidance was issued by the Department for Communities and Local Government in September 2016 and is directed at those responsible for preparing LPGS investment strategy.
The PSC has asked a judge to rule that the guidance is legally flawed as one section prevents pension funds set up under the scheme from engaging in boycotts and the "ethical divestment" of companies accused of being complicit in Israel's occupation of Palestine.
Campaigners say the illegal occupation is 50 years old this month and people have a right to decide not to profit from human rights abuses.
Their counsel, Nigel Giffin QC, told the judge, Sir Ross Cranston, who will give his decision on Thursday, that it was the Government case that boycotts were contrary to UK foreign policy.
He added: "We say the secretary of state (Sajid Javid) has acted contrary to English and EU law and the guidance is marred by hopeless uncertainty as to what he actually intends."
Government lawyers argue that all the grounds of challenge lack substance and the campaigners' application for judicial review should be dismissed.
PSC chairman Hugh Lanning said before the hearing: "We have high hopes that Theresa May's Government will have more egg on its face after this court case.
"Everyone has a right to peacefully protest Israel's violation of Palestinian human rights. It is reprehensible to forbid people from making decisions about where their own money goes, and forcing them to profit from human rights abuses."
The LGPS is a statutory superannuation scheme. Across the country, there are some 80 discrete funds into which members and their employers contribute and each fund has its own administering authority.