Mr Cameron made his vow in the Rochester and Strood constituency ahead of a by-election caused by the defection of former Tory MP Mark Reckless to Ukip.
But the so-called “emergency brake” on EU immigration has been dismissed by opponents as “blind panic”, as support seeps away from the Tories to Ukip.
Mr Cameron portrayed his move as “one last go” to make membership of the European Union work for the UK.
He made clear in his party conference speech that immigration and challenging free travel between EU states would be at the heart of his renegotiation of Britain’s membership.
It is understood he supports an Australian-style points system for EU immigrants to be allowed to work in the UK.
He said yesterday: “It is one of the most important issues in this election. We will be setting out further steps in the weeks ahead.”
The pressure is mounting on the Prime Minister with senior Tory backbench sources saying that if he loses Rochester and Strood on 20 November, he will face a vote of no confidence from his party.
All Tory MPs have been instructed to visit the constituency at least three times during the campaign.
One senior figure told The Scotsman: “Cameron has told us we will throw the kitchen sink at this by-election. If we lose it, what have we got to lose with a vote of no confidence?”
In a question and answer session yesterday, Mr Cameron said that he had found Mr Reckless’s decision to switch his allegiance “baffling” adding: “He was elected on a Conservative ticket and he chose to change sides. He has let people down.”
Mr Cameron also introduced the two Conservative candidate contenders, local councillors Kelly Tolhurst and Anna Firth, who will be chosen in a poll open to all voters in the constituency, regardless of party affiliation.
On Europe, he admitted that negotiating a new deal for Britain would not be easy, but he insisted that it was worth a try.
“We need to get back to what we thought we were joining in the first place,” he said.
Ukip described the briefing and Mr Cameron’s comments as “hot air”.
Migration spokesman Steven Woolfe claimed that Mr Cameron is “running scared”.