LONDON: Leaving the European Union would risk Britain's economic and national security, David Cameron will say in a speech tomorrow (Tuesday).
The Prime Minister will say that an EU departure is "not just a matter of jobs and trade but of the safety and security of our nation".
The comments risk being dismissed as scaremongering by Eurosceptics.
In his Daily Telegraph column today, Boris Johnson - the London Mayor tipped by some to want to lead the "out" campaign - warns against listening to those predicting disaster if Britain leaves the EU.
Mr Johnson forecasts "torrents of drivel" from "Europhiles in Parliament" about the dangers.
Mr Cameron's speech will kickstart the final stage of his attempt to reshape Britain's relationship with Brussels ahead of an in-out referendum.
Mr Cameron is to write tomorrow to Donald Tusk, the European Council President, setting out reforms which Britain would like to see. The letter is expected to make clear that the UK wants opt-outs from ever-closer union, more powers to block EU laws and no further interference in British trade.
It will also favour restrictions on benefits for EU migrants, although Philip Hammond, the Foreign Secretary, warned yesterday that a proposal to stop payments for four years could be illegal.
In his speech Mr Cameron will say that the vote - which must be held within the next two years - will be "a huge decision for our country, perhaps the biggest in our lifetimes".
He will say: "You, the British people, will decide. At that moment, you will hold this country's destiny in your hands." He will also formally rule out any chance of a second referendum after the in-out vote, as has been mooted by some Eurosceptics. The referendum will be "the final decision".
He will add: "No one doubts that Britain is a proud, successful thriving country. A nation that has turned round its fortunes though its own efforts. A far cry from the 'sick man of Europe' at the time we entered the European Economic Community four decades ago.
"Whether we could be successful outside the European Union is not the question. The question is whether we would be more successful in than out? Whether being in the EU adds to our economic security or detracts from it? Whether being in the EU makes us safer or less safe? That is a matter of judgment."
In his column, Mr Johnson says: "We will hear absolute torrents of drivel from all the people who traditionally warn about us not being included in some European project, big business, the CBI, the Europhiles in parliament.
"Remember: they were wrong about the ERM [European Exchange Rate Mechanism], they were wrong about the euro, and they are already exaggerating the downsides of leaving."