There is sufficient evidence to investigate alleged Israeli and Palestinian war crimes committed in the West Bank, East Jerusalem and the Gaza Strip, the international criminal court has announced.

In a landmark decision, the ICC said it saw “no substantial reasons to believe that an investigation would not serve the interests of justice”.

The announcement ended years of preliminary investigations into alleged crimes by both Israeli forces and Palestinians, and signalled that the court was preparing to open a formal investigation.

A statement published by the court’s chief prosecutor, Fatou Bensouda, on the court’s website on Friday said her office “has concluded with the determination that all the statutory criteria under the Rome statute for the opening of an investigation have been met”.

“In brief, I am satisfied that war crimes have been or are being committed in the West Bank, including East Jerusalem, and the Gaza Strip”.

Bensouda’s statement followed a legal opinion published earlier on Friday by Israel’s attorney general, Avichai Mandelblit, arguing that the court had no jurisdiction in the West Bank or in Gaza – apparently intended to pre-empt an imminent decision.

Despite the unprecedented statement by the court, which had been initially petitioned by the Palestinian Authority, Bensouda added that she had referred the question back to the court to “confirm” the court’s jurisdiction over the occupied Palestinian territories.

“I have sought confirmation that the ‘territory’ over which the court may exercise its jurisdiction, and which I may subject to investigation, comprises the West Bank, including East Jerusalem, and Gaza,” she said.

“Such determination is made strictly for the purposes of determining the court’s ability to exercise its jurisdiction and the scope of such jurisdiction under the statute,” Bensouda added.

The ruling was immediately condemned by the Israeli prime minister, Benjamin Netanyahu, who described it as a “black day for truth and justice” and a “baseless and scandalous decision”.

The Palestinian Authority, however, welcomed it in a statement demanding proceedings go forward without further delays.

“After nearly five years of preliminary examination, the Palestinian people who seek redress in this court expect actions congruent with the urgency and gravity of the situation in Palestine and they rightly demand that these steps are taken without delay.”

The full ruling by the court details the “reasonable basis to believe that war crimes were committed in at least three incidents during the 2014 Gaza war” by members of the Israel Defence Forces (IDF), including “intentionally launching disproportionate attacks”.

More widely, the ruling argued that the court believed there was a reasonable basis to believe that Israel had “committed war crimes … in relation to the transfer of Israeli civilians into the West Bank since 13 June 2014.”

It added that the scope of any formal proceedings might be expanded to include “an investigation into crimes allegedly committed in relation to the use by members of the IDF of non-lethal and lethal means against persons participating in demonstrations beginning in March 2018.

The investigation also identified evidence to support claims of war crimes against Palestinian armed groups, including Hamas.

“In addition, there is a reasonable basis to believe that members of Hamas and Palestinian armed groups committed the war crimes [including] intentionally directing attacks against civilians and civilian objects and using protected persons as shields; wilfully depriving protected persons of the rights of fair and regular trial and wilful killing or torture and/or inhuman treatment and/or outrages upon personal dignity.”

The question of jurisdiction is likely to be fiercely contested. Under the Rome statute that established the court, it can hear cases only if one of the parties is a signatory. While Israel, like the US, is not a party to the convention, the court accepted Palestinian membership in 2015.

While the prosecutor’s office has argued that its “primary position” is that Palestine is a state for the purposes of the treaty that established it, in his legal opinion on Friday Mandelblit argued that because Palestine was not a sovereign state the court had no jurisdiction.

“The claim that the Palestinians have purported to join the Rome statute does not meet, nor can it replace, the substantive test requiring criminal jurisdiction to have been delegated to the court by a sovereign state with a defined territory,” he said.

A full ICC investigation could lead to possible charges being brought against individuals. States cannot be charged by the ICC.

The nearly five-year preliminary investigation has looked at the 2014 war, which left 2,251 dead on the Palestinian side, the majority of whom were civilians, and 74 on the Israeli side, most of them soldiers.

It has also looked at violence near the Israel-Gaza border in 2018.

Earlier this month, the ICC prosecutor refused to press charges over a deadly 2010 Israeli raid on a flotilla bringing aid to Gaza, and urged that investigation to be shut.

Nine Turkish citizens died in May 2010 when Israeli marines stormed the Mavi Marmara, which was among eight ships trying to break a naval blockade of the Gaza Strip. One more died in hospital in 2014.