Immigrants from European Union (EU) countries cost the UK public purse nearly £400 million a year, according to campaigners.
Migration Watch UK has drawn up the figures in response to a European Commission (EC) report that found so-called benefits tourism was " 'neither widespread nor systematic".
The report revealed that around 41,000 EU foreign national jobseekers have never worked in the country.
Based on claiming both Jobseeker's Allowance and housing benefit, Migration Watch has calculated this to come to £400 million a year, excluding child benefit, child tax credit, as well as education and health service costs.
Sir Andrew Green, Migration Watch UK chairman, said: " Unlike our European partners the UK benefits system is wide open to those who have never contributed. A determined renegotiation is now essential to ensure those who have made no contribution should have no access to benefits."
The EC report found a sharp 42% rise in numbers of economically non-active EU migrants in the UK, from 432,000 in 2006 to 612,000 last year.
There was a 73% increase in EU migrants coming to the UK to seek work between 2009-11, it said.
But it found that migrants from the eastern European states which joined the EU in 2004 had made a ''positive'' contribution to the UK's finances, paying 37% more in taxes than they receive in services and benefits.
NHS spending of 1.8 billion euro (£1.5 billion) on care for non-active EU migrants amounted to 1.1% of the total health service budget or 0.1% of UK national income, the report said. Meanwhile, some 2.6% of those claiming Jobseeker's Allowance (JSA) were EU migrants.
European employment commissioner Laszlo Andor, who commissioned the report, said: ''The study makes clear that the majority of mobile EU citizens move to another member state to work and puts into perspective the dimension of the so-called benefit tourism, which is neither widespread nor systematic."
But Prime Minister David Cameron's official spokesman said there was ''widespread and understandable concern'' among the public that benefits were among the ''pull factors'' that persuade migrants to come to the UK.
He defended UK Government action against welfare tourism, including plans to tighten the ''habitual residence test'' which determines whether immigrants from the EU can claim benefits such as JSA, as well as an audit of the cost to the NHS of treating EU nationals.
''The Government's view very clearly is that changes do need to be made in this area,'' said the spokesman.
The 600,000-plus economically non-active EU migrants in the UK include not only jobseekers but students, pensioners, disabled people and stay-at-home spouses of workers.
The European Commission report found that, across the EU as a whole, European citizens living in different members states use welfare benefits ''no more intensively than the host country's nationals'' and are less likely than native populations to receive disability and unemployment benefits in most countries studied.
The study found that the vast majority of EU nationals moving to another EU country do so to work, and that EU migrants aged over 15 in the UK were more likely to be in work than British nationals.