Ministers risk raising false expectations of restricting migrants' rights to benefits in a bid to restrict the numbers arriving from Bulgaria and Romania when controls lapse at the end of the year, Ed Miliband warned on Sunday.
The Labour leader said the Government should concentrate on action against rogue employers that exploited cheap labour rather than "windy rhetoric" about challenging European Union (EU) rules on welfare claims.
He spoke out as he left the UK for a three-day tour of Denmark, Sweden and Holland for talks with political leaders about the EU and the economy.
Prime minister David Cameron told ministers last week to prepare action to ensure that the UK's public services and welfare system are not a "soft touch" for migrants, including those from other EU states.
He chaired a Whitehall committee on the issue and is reported to be holding another high-level policy meeting on the issue later this week amid fears of a potential wave of immigrants from the EU's newest member states.
Work and Pensions Secretary Iain Duncan Smith declared he was braced for a "big battle" with Brussels over the rights of jobless immigrants to claim benefits and send some of their cash back to families in their home nations.
Britain already had strong allies in the push - including the Dutch and Scandinavian countries - he told BBC1's Andrew Marr Show, suggesting it would be a "critical" part of Cameron's bid to return powers from the EU.
"We have a number of countries on our side so I think we will be able to tighten up and make those regulations much tougher for people coming in just to take advantage of our benefits system," he said.
Asked about the tough messages, Miliband told BBC Radio 5Live's Pienaar's Politics: "I think that the way you should always approach these issues is talk about what you can do, don't start floating things unless you know that they are actually possible to be done.
"We said around the issue of Romania and Bulgaria of course we should look at the benefits issue and access to benefits. All of those issues should be looked at but let's look at what can be done because I think it's very, very important that you don't raise people's expectations and then find that you can't actually make a difference.
"One very specific thing the Government should be doing before Romanian and Bulgarian accession in terms of the openness of borders is to say 'let's make sure we clamp down on rogue employers; rogue employers who bring people in and pay less than the minimum wage, who have ten, fifteen people in a house'.
"All of those things undercut workers already here and exploit workers coming here. I think what we need is action from the Government, rather than just windy rhetoric about what they are going to do."
Duncan Smith said it was "pointless" trying to estimate the numbers who would arrive from Bulgaria and Romania given how far wrong the previous government turned out to be after the last round of EU accessions.
Accompanied by shadow foreign secretary Douglas Alexander, Miliband is due to met Danish prime minister Helle Thorning-Schmidt tomorrow before travelling on to Sweden and then Holland.
"I will be talking to allies across Europe about how we change it to make the EU work for working people and help us all begin building for the future," he said.
"And learning lessons for Britain about how we can create a successful economy, one which is made to last."
Suggested For You
Get top stories and blog posts emailed to me each day. Newsletters may offer personalized content or advertisements. Learn more