Parliament will debate a motion that will allow British military action in the form of air strikes on specific targets within Iraq.
If approved, strikes could begin as soon as this weekend.
The motion says: "This House condemns the barbaric acts of ISIL [IS] against the peoples of Iraq including the Sunni, Shia, Kurds, Christians and Yazidi and the humanitarian crisis this is causing.
"[This House] further acknowledges the request of the Government of Iraq for international support to defend itself against the threat ISIL poses to Iraq and its citizens, and the clear legal basis that this provides for action in Iraq."
IS has killed thousands of Christians and forced many from their homes.
In some towns there are no believers left at all.
The militants have previously issued an ultimatum to Christians to pay a tax, convert to Islam or face death.
The prime minister has described IS as an "evil against which the whole world must unite".
David Cameron added the UK was ready to "play its part".
Speaking to Premier earlier in the week, former defence minister and Christian MP Sir Gerald Howarth said he would back the vote.
He said: "The strikes by the United States have been instrumental in assisting those on the ground to resist the advance of IS, which threatens to engulf the whole of that country.
"If it succeeded, it would unquestionably cause massive instability in the region and affect us here at home."
More than 190 air strikes against IS in Iraq have been carried out by America since August.