Thousands of migrants from the newest EU member states are homeless in the UK and Ireland after failing to find jobs or access benefits, a report indicates.
The number of rough sleepers in London doubled, the report says
An estimated 3,000 Poles are living rough in London, the European Federation of National Organisations Working with the Homeless report says.
A "proportionately similar" number were in Dublin and in Scotland, the European non-governmental group added.
It said the results were a warning to other countries.
The UK and Ireland were two of three of the then 15 member states that allowed immediate full access to labour markets for citizens from the 10 countries that joined in 2004.
Other EU countries opted for a phase-in transition period.
In a statement, the European Federation of National Organisations Working with the Homeless (Feantsa) said: "Workers from the new member states have become particularly vulnerable to homelessness.
"Some of them could not find suitable employment or lost their jobs.
"They have become homeless because they have no income and are not entitled to social security benefits during the initial years of their stay in the host country.
"Similar developments are likely to occur in other European countries that recently opened or will soon fully open their labour markets to workers from the new member states."
Feantsa said the "vast majority" of migrant workers were making a significant economic contribution in their host country.
But the statement added: "Some people have not been able to find proper employment immediately.
"Many are employed in difficult circumstances and are under-paid.
"Their precarious employment situation has reduced their options on the housing market considerably.
"People from new member states are one of the fastest growing groups amongst users of food handouts and in some places also homeless hostels.
"Many of them live in very inadequate housing such as squats, or are sleeping rough.
"In London, the number of people sleeping rough doubled over the last 12 months."
A new group of long-term homeless people would emerge unless urgent action was taken, Feantsa warned.
"The EU should ensure that all EU migrant workers have access to the same rights and benefits as nationals," the statement said.
"The free movement of workers, which is one of the fundamental freedoms of the EU, must not become a poverty trap for EU citizens who are hoping to find a better life abroad."
The warning comes after the EU agreed to admit Bulgaria and Romania from the start of next year.
Home Secretary John Reid has said the UK is likely to restrict job-seekers from the two countries.