The House of Commons has voted unanimously to pass a historic motion labelling ISIS' atrocities against Christians, Yazidis and other religious minorities as 'genocide'.
However the government refused to support the motion and instructed all ministers and ministers' aides to abstain from the vote that followed the three-hour debate.
After an emotive debate the remainder of backbench MPs voted unanimously by 278 to 0 in favour of the motion which called on the government to make a referral to the United Nations Security Council (UNSC).
Foreign office minister Tobias Ellwood repeated the government's position which is that genocide is a matter for the international legal system, and not governments.
However the Christian MP Fiona Bruce, who tabled the motion, called this a "circular argument" because the International Criminal Court (ICC) cannot intervene until it is instructed to do so by the UNSC, of which the UK is a permanent member.
In his response to the debate Ellwood said the last time the UNSC referred a case to the ICC it was vetoed by Russia. However Bruce retorted this was in relation to the Assad regime which the Kremlin supports and not ISIS, which has no support on the council.
David Burrowes, Conservative MP for Enfield Southgate, also criticised the government's position in the debate. He told Christian Today: "This was an overwhelming vote in favour of the motion. It sends a very clear unanimous voice of the House of Commons that the government needs to take action.
"This issue will not go away. The payroll was ordered to abstain from the vote and no doubt many of them would have supported the motion as well."
On the government's response he added: "Just because there is a prospect of a veto it does not mean you do nothing.
"It is about the principle. Once the threshold of genocide has been reached, as it clearly has, we have an obligation."
Despite calls for a free vote, nearly half of Tory MPs were barred from voting after whips told all MPs on the government payroll to abstain. A source close to the campaign in favour of declaring genocide told Christian Today this was "constitutionally dodgy" as it prevented around 40 per cent of Conservative MPs from voting.
MPs from all major parties spoke in favour of the motion including Labour MP for Ealing North Stephen Pound, who said: "If this is not genocide then what is genocide?"
Labour's faith envoy Stephen Timms, MP for East Ham, said it was "difficult to deny" ISIS' atrocities amounted to genocide and Nusrat Ghani, the Tory MP for Wealden, added: "If we don't recognise these acts as genocide we effectively say we are not willing to take all actions necessary to bring them to an end."
After the debate Bruce said it was "right" parliament had passed the motion.
"It is now the Government's responsibility to make a referral to the UN Security Council as soon as possible, and we look forward to the Foreign Office confirming its intention to do this in the very near future," she added.
The vote comes after the European Parliament passed a similar motion unanimously in February and the US secretary of state John Kerry declared ISIS was "genocidal" in March.