Amal Clooney and Boris Johnson will back a plan to collect evidence of ISIS' "abhorrent crimes" in the hope of one day bringing perpetrators to justice.
The move expected at the United Nations meeting in New York on Monday would urge governments and NGOs to join a campaign to assemble evidence of crimes. A purported lack of evidence has been hailed as one reason not to declare ISIS' treatment of Christians, Yazidis and other minorities a genocide.
Before his appointment as foreign secretary, Johnson repeatedly called for ISIS' crimes to be labelled a genocide. But since his promotion to the foreign office, a department traditionally very reluctant to use the term, Johnson appears to have softened his stance.
The collaborative move with Clooney comes months after MPs voted unanimously to refer the case to the United Nations security council. In response, the foreign office minister Tobias Ellwood said the government would not do that because Russia, another permanent member of the security council, would veto the resolution.
Ahead of the meeting Johnson said: "We have united to defeat Daesh, now we must unite to bring them to justice. That needs to include looking at ways for the UN to support the vital task of gathering evidence about their abhorrent crimes.
"The global campaign will seek justice for all Daesh's [ISIS'] victims, bringing the international community together in defiance of Daesh's efforts to stoke division and hate."
Clooney has been more consistent in her campaign for genocide to be declared at a UN level. On Friday the international human rights lawyer lambasted the organisation and said she was "ashamed" of its failure to "to prevent or even punish" ISIS for its crimes.
In an address to the UN she spoke of her shame that "not a single member of ISIS has been prosecuted in a court anywhere in the world for crimes committed against the Yazidi" people.
Clooney is set to act as counsel for the Yazidi community at the International Criminal Court (ICC), where investigations into possible genocide usually take place. But in order for the ICC to act, it must be instructed to move by the UN security council.
Robert Clarke, Director of European Advocacy for ADF International, an organisation who have campaigned for the term genocide to be applied, condemned the UN for being "reluctant to act". He described Clooney's speech as "helpful" but said: "While Yazidis continue to suffer greatly at the hands of the terror regime, so too do Christians and other religious minorities. All victims deserve justice, and all perpetrators must be held to account."
He added the UK's unwillingness to use the term genocide, which has significant legal and moral ramifications, has led to "doubt in the face of irrefutable evidence".
He told Christian Today: "We hope that this increasing engagement at an international level marks a change, and that the UK will use all its influence to ultimately stop the killing."
