THE HAGUE (Reuters) - The Palestinian Authority becomes the 123rd member of the International Criminal Court on Wednesday, a major step that could move its decades-long conflict with Israel into a courtroom.

Israel is opposed to the court, is not a member and has no plans to cooperate with investigators already looking into possible crimes by both sides during fighting. It was furious when the Palestinians announced their application on Dec. 31 and tried to undermine it by lobbying to cut funding to the court.

But the Palestinians’ membership poses a major dilemma for Israel. Israel may be stuck on the legal sidelines unless it makes a major policy shift by cooperating with or even joining an institution born out of the principles of the Nuremberg trials of Nazi leaders.

“The ICC can issue summons to appear in court and arrest warrants. It will say a lot about a country if it doesn’t respond,” a Palestinian official said.

“The panic attacks we’ve seen (from Israel) suggest we are not the only ones to take the court seriously.”

Israeli officials say they will wait to see what happens once the Palestinians have joined the court before commenting.

The benefits of signing up to the treaty that governs the court include diplomatic immunity for government officials working with the court, fast-tracked reviews of cases it puts forward and the right to nominate judges.

From April 1, the court will have automatic jurisdiction over any crimes committed in the territory of the Palestinian Authority, or by its citizens. It has also already received permission to make a preliminary examination of events since June 13, 2014, shortly before Israel began a military offensive in Gaza. Several Palestinian-linked NGOs are expected to submit legal arguments and dossiers of evidence to the court in the coming days or weeks, but the state of Palestine is not expected to make a formal referral to the prosecutor, which would trigger the appointment of a panel of judges.

SETTLEMENTS CONTESTED

An Israel-Palestine case, experts believe, would examine battlefield conduct by both Israeli troops and Palestinian militants in Gaza, but could also consider the Israeli settlements on occupied land wanted by the Palestinians.

Palestinians seek a state in Gaza, the West Bank and East Jerusalem, lands Israel captured in 1967. Most countries view Israel’s settlement building on occupied land as illegal.

On the flipside, from the moment the Palestinians join, prosecutors will gain the right to investigate any alleged crimes on the Palestinian side, even against the will of the Palestinian leadership.

That risk was highlighted last week by an Amnesty International report detailing alleged human rights violations by Palestinians against Israelis during the Gaza conflict.

Experts said Israel could not block a case from being launched, but it could stall the process to trial by starting its own inquiries into the conduct of its soldiers, stonewalling investigators, and ultimately refusing to hand over suspects.

Israeli officials say Palestinian membership, which has also drawn criticism from the United States, another non-member of the ICC, undermines the chance of peace.

But their opposition to the court has become less outspoken as they wait for guidance from the new government of re-elected Prime Minister Benjamin Netanyahu and consider the disadvantages of being left out of the legal process.

Prosecutors conducting the preliminary inquiry of alleged crimes committed during the Gaza conflict have a fine line to tread examining publicly available information to assess whether a full criminal investigation is warranted.

“If we come up with a decision too quickly we’ll be accused of paying Israel special attention,” said one senior prosecution official. “But if we take too long we’ll be accused of being afraid of Israel or the United States.”

SLOW JUSTICE

Few expect a quick decision. In the 13 years since the court was set up, to investigate war crimes or crimes against humanity in cases where local authorities are unwilling or unable to deal with them, the time between a preliminary examination and the opening of a full investigation has often taken over a year.

Prosecutors have examined situations in 20 countries and opened investigations in just eight of them. The court has concluded only three cases, securing two convictions.

Prosecutors are obligated to examine any material submitted to the court by any party and could then ask the parties involved to make legal submissions.

A refusal by Israel to cooperate could thwart any attempt by the court to investigate or charge Israeli citizens, a problem it faced with investigating Kenya’s President Uhuru Kenyatta.

In their highest-profile case until now, prosecutors admitted defeat after alleging that their witnesses had been threatened and that Kenyan authorities had denied them access to crucial records.

In any case dealing with Israel and the Palestinians, experts expect the court to seek out alleged crimes that offer scope for charges to be brought against both sides.

“If prosecutors do open a formal investigation, they’re going to be very wary of choosing an area where they’d only investigate Israel,” said Kevin Jon Heller, professor of law at London’s School of Oriental and African Studies.