The MPs and peers urge Cameron to use his influence to reach an agreement with the UN that the term genocide be used.
The letter reads - "This is not simply a matter of semantics. There would be two main benefits from the acceptance by the UN that genocide is being perpetrated.
"First, it would send a very clear message to those organising and undertaking this slaughter that at some point in the future they will be held accountable by the international community for their actions; they will be caught, tried and punished.
IS has been systematically killing minority groups including Iraqi and Syrian Christians and Yazidis, they said.
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The UN has cited the Yazidis' treatment by IS as evidence that it may have committed genocide and war crimes in Iraq.
Over the past year, ISIS militants have kidnapped thousands of young Yazidi women to use as sex slaves.
The terror group has also been trying to eradicate minority groups from large parts of the country, international human rights groups have warned.
Article II of the 1948 UN Genocide Convention defines genocide as any act committed with intent to destroy, in whole or in part, a national, ethnic, racial or religious group, such as killing members of the group, deliberately inflicting conditions of life calculated to bring about its physical destruction in whole or in part, imposing measures intended to prevent births within the group, or forcibly transferring children of the group to another group".
IS first emerged in Syria fighting President Bashar al-Assad during the ongoing civil war there.