Tony Blair has dismissed Conservative claims his government is in "total confusion" over immigration controls when 10 new countries join the EU.
Blair's talk of "other measures" over migrants has caused speculation
Tory leader Michael Howard said temporary controls should be put on migrants after EU enlargement.
He argued Mr Blair had pledged such controls and a "climb-down" followed.
The prime minister said there would be moves against benefits abuse as well as "other measures" - a comment which has led to more speculation.
All European Union citizens can work in Britain without work permits - prompting suggestions there will be an influx when the poorer new states join this year.
Britain and Ireland are the only existing EU members who have decided not to impose transitional arrangements after EU enlargement.
Some newspapers have raised fears about an influx of Roma people
At prime minister's questions, the Tory leader tackled Mr Blair for the second week running on the issue.
He claimed Mr Blair had indicated he was considering a work permits system only for officials and the home secretary to take a different line.
A "bold statement" to grab headlines had been overtaken by a climb-down and "total confusion" by last weekend, he claimed.
But Mr Blair denied making any such commitment.
EU laws allowed the free movement of people, but not of workers, he said.
Britain was currently prepared to grant a concession allowing free movement of workers to Britain too, he said.
But first "we have to make sure that any potential basis for exploitation or loopholes in the rules is closed off. That is what we are looking at now, that includes obviously looking at the benefits regime", he said.
As well as benefits, there could be "other measures", he added, but Downing Street later did not enlighten journalists on what those could be.
The prime minister's spokesman did not rule out the possibility of taking further steps under the transitional arrangements for the new EU countries.
The government was looking at possible "safeguards", he said.
Asked about a possible influx of immigrants, he said the government was "live to the possible problems - should they manifest themselves, we want to be able to take appropriate action".
Work and Pensions Secretary Andrew Smith is due to announce the measures to prevent benefits being exploited.
But the announcement appears to have been delayed in a sign the government is facing difficulties over the details of the plans.
Home Secretary David Blunkett has argued people from new EU member states will be able to come to Britain as visitors in any case.
If they have to get work permits, there is a danger they could disappear into black market jobs, he says.
There are also key labour shortages in areas like Scotland, says Mr Blunkett.
On Thursday, shadow home secretary David Davis told BBC Radio 4's Today that there was no problem with people filling genuine shortages.
But he added: "What we are talking about at the moment is complete, uncontrolled, open access."
But Heaven Crawley, from leading left-leaning think tank the Institute for Public Policy Research, said estimates suggested there would be 250-300,000 people moving for the whole EU over the next 15 to 20 years.
"There is clearly a mismatch between the debate and the reality about what the evidence tells us is going to happen," she said, urging ministers to stick to their guns on work access.