George Osborne appears to have admitted defeat on the Tory pledge to bring annual net immigration below 100,000 by the general election.
The Chancellor said progress had been made on reducing numbers, but suggested Britain's relationship with the EU would need to be renegotiated after May 2015 to deliver David Cameron's promise.
He also warned that border controls will be loosened again if Labour returns to government next year.
The comments came after the Prime Minister and other senior ministers repeatedly feathered questions about whether the goal can be met.
Official figures showed net migration - the number coming to the UK for at least a year, minus the numbers leaving - rose 58,000 to 212,000 in the year to September 2013.
Mr Cameron has rejected calls to drop the target, arguing it is still "achievable" but refusing to offer a "cast iron guarantee". Home Secretary Theresa May has conceded it has become "more difficult".
Interviewed for the Sun on Sunday, Mr Osborne suggested that the EU membership renegotiation - due to take place if the Tories win the general election - will have to be completed in order to bring immigration down to the tens of thousands.
"We've set out our ambition, we are committed to that ambition, and as you see with the Immigration Act now coming into effect we are delivering the actual changes that will help us control our borders and deliver that reduction in numbers that people in this country want to see," Mr Osborne said. "So we have got our policy, we are delivering on the policy, and the key dimension to it which we need to now deliver on is the European aspect.
"That requires renegotiation of our membership of the EU, an in-out referendum so the British people have their say.
"The point that people need to focus on is that a general election is not a free hit, it matters who the Government is.
"If there's a Labour government not only will the economy go to ruin but the borders will be uncontrolled and you won't get that renegotiation in Europe because they're not even pretending that they want to do it."
On the by-election campaign trail in Newark, Mr Osborne insisted: "There needs to be a different set of rules than the ones Labour signed us up to when those countries in Eastern Europe joined, when there were no restrictions.
"We are saying, 'Look, for new accession countries you need to look at how free movement works.'"
Speaking to the Sunday Telegraph, Iain Duncan Smith also stressed the Government had made progress on immigration - which has been identified as one of the main factors in Ukip's strong performance in local and European elections.
But the Work and Pensions Secretary said "there remains an issue over Europe".
"Freedom of movement means that people can come in an out. Particularly now you have countries with lower GDP, there is a big pull factor to come into countries like ours," he said.
Mr Duncan Smith called for new rules to limit migration from the EU, and said Brussels should be stripped of control over who is entitled to state benefits in Britain.