Migrants from the countries joining the EU in May will have to work or leave the UK, Tony Blair has said.
Blair talked tough during radio phone-in
Residents of the eight former East European communist countries will only be allowed to stay in Britain if they can support themselves, he said.
He spoke before David Blunkett outlined to MPs what measures are planned to deal with potential migrants.
"If they can't support themselves, they will be put out of the country," the prime minister told BBC WM.
Mr Blunkett is expected to seek to welcome workers who fill gaps in the UK labour market, but wants to prevent "benefit tourism".
The measures were agreed at a hastily arranged meeting in Downing Street chaired by Mr Blair last week.
Ministers have denied the package was put together in a last-minute panic.
There is widespread speculation the measures will include some kind of register of those seeking employment.
It has been suggested that work permits may be issued, but that has not been confirmed.
The length of time someone has to live in the UK before becoming eligible for income support, housing and council tax benefit is also likely to be extended.
The habitual residence test could be tightened, to ensure benefits are paid only to those who are genuinely settled in Britain.
But Mr Blair insisted: "What we'll be saying is people can't claim benefits here and they can only come, therefore, if they have got a job to do.
"If they can't support themselves, they will be put out of the country.
"We will be making it clear that ... you can come if you are coming to work and you have got a job to do, but you're not going to be able to claim benefits."
Britain and the Irish Republic are the only two EU countries still planning to give the unrestricted right to work to people from states like Poland and the Czech Republic from 1 May.
That has prompted fears there will be an influx of thousands of people.
Tory home affairs spokesman Humfrey Malins told BBC Radio 4's Today programme the government had had three years to put in place transitional arrangements.
"Whilst we don't know how many people may come, the fact is it is an open-ended commitment and this government has actually done nothing to prepare us for it," he said.
The Liberal Democrats say ministers should stick to their guns on not using the permits.
All the new EU citizens will have the right to travel to Britain and Mr Blunkett argues that imposing work restrictions could encourage black market labour.
Migrants will have to work or face deportation, says Blair
There are also gaps in the British labour market which need filling, he says.
All existing member states are able to impose transitional restrictions for up to seven years on the right of residents of eight of the 10 new EU member states to work and claim benefits.
The idea is that by the end of that period the economies of the new members will have grown, making it less likely that mass migration will follow.
The restrictions only cover the former communist states joining the EU.
Ahead of Monday's speech, Mr Blunkett said UK border controls may be extended to Belgium in the drive against asylum system abuses.
He said the government soon expected to replicate a deal struck with France, whereby British officials are allowed to check the documents of passengers boarding London-bound Eurostar trains in Paris.
The first citizenship ceremonies are expected to go ahead this week, even though the government is not yet ready to introduce its planned citizenship tests.
The process will include an oath of allegiance to the Queen and the national anthem will be sung.