The lawmakers urged Mr Cameron to use his influence to reach an agreement with the UN that the term genocide be used.
The letter said that this would send the message that those responsible would be caught, tried and punished.
The MPs claimed that ISIS had been systematically killing minority groups including Iraqi and Syrian Christians and Yazidis.
The UN has cited the Yazidis’ treatment by ISIS, as evidence that the Jihadist group may have committed genocide and war crimes in Iraq.
The Jihadist group has also been trying to eradicate minority groups from large parts of the country, human rights organisations have warned.
The letter, written by MPs Rod Flello and David Alton, said there was clear evidence of ISIS assassinations of church leaders, mass murders, torture, kidnapping for ransom in the Christian communities of Iraq and Syria and “the sexual enslavement and systematic rape of Christian girls and women”.
It also said that the group was carrying out “forcible conversions to Islam”, the destruction of churches, monasteries, cemeteries, and Christian artifacts and theft of lands and wealth from Christian clergy.
The letter read: “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.
“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.
“It would also encourage the 127 nations that are signatories to the convention to face up to their duty to take the necessary action to ‘prevent and punish’ the perpetrators of these evil acts”.