British Prime Minister David Cameron | EPA/ANDY RAIN / POOL Cameron won’t rule out UK exit The UK prime minister will put forward his demands for EU reform Tuesday.

David Cameron will warn EU leaders this week that he may be forced to back the Out campaign if his demands for reform of the U.K.'s membership in the EU are not met.

The British prime minister will formally set out his demands for reform on Tuesday in a letter to Donald Tusk, the president of the European Council. This will mark the start of months of negotiations ahead of the U.K. In/Out referendum before the end of 2017.

According to the Guardian, Cameron is expected to say that the status quo is not acceptable and that his government's stance ahead of the referendum is not predetermined. If the EU does not give "substantial ground" the U.K.'s withdrawal from the EU could follow, says the report.

In a speech on Tuesday, according to the Guardian, Cameron will say that: “If we can’t reach such an agreement, and if Britain’s concerns were to be met with a deaf ear, which I do not believe will happen, then we will have to think again about whether this European Union is right for us. As I have said before — I rule nothing out.”

In his letter Cameron is expected to spell out in general terms the four areas where he wants change: a four-year ban on EU migrants claiming in-work and other benefits; greater protection for non-eurozone countries to ensure they cannot be outvoted by eurozone countries; giving Britain an opt-out from the EU’s commitment to “ever-closer union”; and giving parliaments more powers to club together to block EU legislation.

In Tuesday’s speech, Cameron will challenge both the In and Out campaigns to clarify their arguments. According to the Guardian report, he will say that, “Those who believe we should stay in the EU at all costs need to explain why Britain should accept the status quo. I am clear that there are real problems with this."