Migrants from the eight countries which joined the EU in 2004 will be able to claim full benefits in the UK for the first time from May.
Estimates in The Times suggest as many as 100,000 migrants could claim tens of millions of pounds in benefits.
But the Department for Work and Pensions insisted strict rules would remain in place to prevent abuse and stop "benefit tourism".
The long-expected relaxation in the rules is down to European law.
Giving migrants from the Czech Republic, Poland, Hungary, Slovakia, Slovenia, Estonia, Latvia and Lithuania - the so-called A8 countries - entitlement to a range of benefits was a condition of the 2004 accession treaty admitting these countries to the EU.
The UK was allowed to apply transition arrangements in the labour market for up to seven years - but those arrangements, which included the Worker Registration Scheme, will be dropped at the end of April.
Migrants from the A8 countries will then be brought into line with migrants from all other EU countries.
They will be able to apply for national government benefits, which in the UK includes jobseeker's allowance, council tax benefit and housing benefit, but under what the Department for Work and Pensions describes as "strict rules".
At the moment, migrant workers from the A8 countries cannot claim out-of-work benefits unless they have completed 12 months of work in the UK.
A DWP spokesman said: "No-one can just come into the UK and start claiming our benefits.
"We have strict rules in place to protect the system from any abuse. For instance, to claim an income-related benefit, a person from the EU will have to pass the Habitual Residence Test alongside all of our other eligibility criteria.
"They will have to prove they have a right to reside here and will then be asked to prove their attachment to the UK; they will have to show an intention to settle here and their reasons for coming to the UK.
"We will be keeping our benefit rules to people from abroad under review to ensure it's secure."
'Number one priority'
Immigration Minister Damian Green said that at the same time that the worker registration scheme ended in the UK, countries including Germany and Austria would open their labour markets to workers from their eastern neighbours.
Only Britain, Ireland and Sweden permitted free access to workers from A8 countries in 2004. It is thought that many will now be attracted to Germany and Austria, which are geographically nearer.
Mr Green said: "Under the terms of the Accession Treaty these restrictions have to be lifted at the end of April, giving A8 nationals the same access to work as other EU nationals. At the same time Germany and Austria will be opening their doors to these workers for the first time.
"This government will apply transitional controls for all new EU member states in accordance with the relevant Accession Treaty as a matter of course in the future.
"We are in the process of delivering major reform to bring immigration down to the tens of thousands with the introduction of a new limit on economic migrants from outside the EU, alongside new proposals to reform other routes of entry, including students, families and marriage."
The DWP said it had no choice but to remain in line with national and international obligations, but has insisted that protecting the benefit system from abuse was its "number one priority".
Poland's Labour minister, Jolanta Fedek, said last month that she did not expect the lifting of labour market restrictions to trigger another wave of emigration from her country but would instead encourage the estimated 300,000 to 400,000 Poles working in neighbouring Germany illegally to become legal.
German economists have predicted about 100,000 people from the A8 countries will come to their country after May, but experts have said they did not expect migrants who had settled in other countries to relocate there in large numbers.
Joachim Moller, director of the Labour Market Institute, at the Federal Labour Office, told the International Herald Tribune: "Even though the German economy is now strong again and in need of workers I just cannot see people just moving around like that."