The threat came at an EU summit in Brussels where he acknowledged that “enlargement” had been one of the union’s greatest strengths and a spur for peace and prosperity.
But he said the system was meant to offer the prize of free movement to workers from new member states, not those just looking for cash handouts.
Days after announcing tighter UK controls on migrants’ financial benefits ahead of an expected wave of Romanians and Bulgarians coming to Britain from 1 January, the Prime Minister was looking for support from other EU leaders to rewrite the rules before any more membership hopefuls receive the green light.
There was not much sign of unity on the issue at the summit, but the Prime Minister said he was launching a debate about an issue that needed to be tackled, adding that rules should be tightened, by treaty change if necessary.
With at least six countries in the queue for membership, Mr Cameron observed that each accession would require the approval of all existing member states: “Accession [to full EU membership] requires unanimity so there is a real opportunity to insist on a new approach.”
The most recent expansion ushered in Croatia earlier this year, with full cross-border rights to settle in any of the other members triggered within the next seven years. The EU accession process starts with Serbia next month, and plans are under way to grant Albania “candidate status” as early as next June.
Mr Cameron said he had opposed immediate status for Albania, and wanted to see the start of a serious debate on a rules shake-up – including consideration of his own idea of only granting full cross-border work and residency rights to citizens of newly added member states once their national prosperity has reached a certain level.
He told a post-summit press conference: “I support enlargement as a huge driver of peace and prosperity but the EU of today is very different from that of 50 years ago. The founding fathers didn’t envisage that enlargement would mean mass migration. We must get back to the principle of granting free movement to workers ready to work hard, not those who are after the best benefit deal.
“It is not supposed to be about free movement for benefit tourism for those who don’t have the means to support themselves. It is about people in work looking for work elsewhere.”
He added: “We must learn the lessons of history. The mistake of the last [Labour] government was to give unfettered immediate access [after major EU enlargement in 2004]. That was a huge mistake. People simply didn’t see that one and half million people would move across Europe.”
Me Cameron admitted there were no precise figures on the likely levels of benefit tourism from Romania and Bulgaria.
When challenged that it was too late to affect imminent migration flows from Romania and Bulgaria, Mr Cameron said: “I don’t accept that: it is important in advance of Bulgarian and Romanian accession to make sure we put in place all the right controls and measures we can.”