The European Union referendum will be a "battle for Britain", David Cameron declared, as he launched the Conservative campaign for a vote to stay in.
The Prime Minister promised a "greater Britain" with benefits to trade, security and the UK's influence on the world stage as he addressed supporters.
But with the Tory party deeply divided on the issue, Mr Cameron warned a Brexit would be a "great leap in the dark".
Addressing a Conservatives In rally in central London, the Prime Minister urged Tory activists to campaign with "as much force as you can" to sell his deal with Brussels on the doorstep.
Setting out what is at stake, he said the referendum was about more than simply the party politics of a general election.
He said: "This is going to be a battle for Britain, a battle for the future of our country, the sort of country that we want for our children and our grandchildren.
"No-one cares more than me about winning the next election in 2020 and the one after that.
"But you can elect in politicians, you can elect out politicians - we don't like that bit, I know, when it happens.
"But this is about what sort of a country we are going to be - how wealthy we can be, successful in a single market, or how much uncertainty you can have outside.
"How safe we can be, working with our partners in the EU in combating crime and terrorism. Or how unsafe we can be outside it.
"How strong we can be with a country that has definitely got its momentum back, the economy is growing - others around the world are looking to us about how to sort out your economy, how to be stronger.
"The crucial point is this: we can be stronger inside that reformed EU."
He added: "We can have a greater Britain inside a reformed European Union, or we can have a great leap in the dark."