David Cameron has given his strongest hint yet that he will campaign for Britain to leave the European Union if his demands for reform are not met.
The British prime minister will set out a shopping list of demands on Tuesday in a letter to Donald Tusk, the President of the European Council.
In a speech on the same day, Mr Cameron will say that if the EU agrees to reform he will campaign for Britain to stay in "with all my heart and all my soul, because that will be unambiguously in our national interest".
But he will add: "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."
This week marks the beginning of formal negotiations about Britain's relationship with the European Union ahead of an in/out referendum on its membership.
Other EU countries have previously complained about a lack of detail from Mr Cameron and asked him to put his demands down in writing.
One-on-one discussions with other EU member states and senior officials will begin a week after Tuesday's long-awaited letter to Donald Tusk.
In his speech, Mr Cameron will say: "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. There are some economic risks if we allow a situation where eurozone countries could potentially spend our money, or where European regulations hold back our ability to trade and create jobs.
"And there are also significant risks if we allow our sovereignty to be eroded by an ever-closer union, or sit by and do nothing about the unsustainable rate of migration into our country.
"But just as those who are advocating staying in the EU at all costs have to answer serious questions, those who think Britain should just leave now also need to think hard about the implications of their arguments.
"What would being outside the European Union mean for our economic security? And what does it mean for our national security?"
In a recent report, the Economic and Social Research Institute (ESRI) said Ireland should expect to suffer losses of up to €3 billion a year if Britain decides to withdraw.
