Boris Johnson and the Leave campaign are "making it up as they go along", David Cameron said as he told the people of the UK they face a "profound" decision about the country's future in the European Union referendum.
Marking 100 days until the June 23 vote, the Prime Minister said the result of the referendum would have an effect on family finances, jobs and the UK's place on the global stage.
He accused the Leave campaign of "taking a risk" with people's livelihoods by advocating a change in the relationship with the UK's trading partners.
London Mayor Mr Johnson has suggested the UK could take on "associate membership" of the EU if it votes to leave, in an arrangement similar to that currently applied to Turkey.
But Mr Cameron accused the pro-Brexit camp of shifting their position on the kind of relationship they envisage for the UK with the EU after a vote to Leave.
Speaking at a campaign event in Felixstowe, the Prime Minister said: "Those people who want us to leave, they cannot tell us what alternative they would put in place.
"If we stay, we know we have unhindered access to that market of 500 million people. The people who want us to go, to start with they said 'We are not going to tell you what the alternative would be', then they said they wanted full access to the single market - but that means you get the free movement of people and you have to pay into that single market without any say about what the rules are - what's the point of that?
"Then they said they want a Canada-style free trade deal, but a Canada-style free trade deal means you don't have full access for your financial services, you have to pay tariffs on your cars, you don't have full access for your farmers' produce, so it's not a great deal for Britain.
"Canada is a country 4,000 miles away from the continent of Europe that does 10% of its trade with the EU. We are a country just 20 miles away from the continent of Europe and we do 50% of our trade with the European Union.
"So a Canada deal is not the right deal for us.
"Today, the leaders of the Leave campaign are saying they don't really want a Canada deal at all, that they weren't right about that. They are literally making it up as they go along.
"They are rolling the dice, they are taking a risk and they are taking a risk with people's jobs, taking a risk with families' finances and I don't think that is good enough for the British people."