Islamic State (Isis) has carried out a campaign of murder, violence and repression against Christians and the Yazidi ethnic and religious minority since seizing large swaths of northern Iraq and Syria.

However, the Foreign Office directed ministers and parliamentary aides to abstain, saying it was wrong for the government to prejudge the issue or act as a jury on a case that may yet be referred to the international criminal court.

The United States Congress, the US administration, the European parliament and the Council of Europe have all declared the terror group’s treatment of the Yazidi community as genocide, but the Foreign Office legal department has a long-standing policy dating back to the passage of the genocide convention in 1948 of refusing to give a legal description to potential war crimes.

The government abstention policy was also designed to minimize the significance and size of the Commons majority, as well as to disassociate the executive from the MPs’ vote.

Tobias Ellwood, the Foreign Office minister, facing jeers and interruptions, said the immediate task was to compile evidence of the harrowing and unspeakable crimes, but it was wrong to declare now the nature of the crimes being committed against the Yazidis. 

He said he personally believed genocide had taken place, but said this was a matter for the courts and not politicians. He added that any referral to the international criminal court required the support of the UN Security Council and that such a reference was blocked by Russia and China in 2014.

“This ultimately is a matter for courts to decide. It is not for governments to be the prosecutor, the judge or indeed jury,” he said.

He insisted that regardless of the precise legal description of the crimes, justice would be brought to bear on those responsible; no matter how long it takes, insisting the government was not washing its hands of the issue.