BBC News Online outlines the issues surrounding the expansion of the European Union and suggestions of mass migration from new member states.
UK was initially not planning restrictions on new EU member states
What is European enlargement?
The European Union's 15 members will increase by a further 10 at the start of May.
Who is joining?
Cyprus and Malta plus the former communist states of the Czech Republic, Slovakia, Poland, Lithuania, Latvia, Estonia, Hungary and Slovenia.
So why the migration worry?
There are suggestions that large numbers of people from the former communist states will be tempted by jobs with higher wages and/or more generous benefits in the existing EU states.
What was the EU reaction?
Although all EU citizens in theory have the right to live, work and claim benefits in any member country, existing members are allowed to limit some of these rights - relating to employment - for residents of the eight ex-communist states for a "transitional" period of up to seven years.
Why only for seven years?
The hope is that after seven years the new members will have overcome the economic divide and there will be no great desire for mass migration.
How have existing members responded?
It varies from country to country and not all have finalised their plans. Germany and Austria are thought likely to impose restrictions for the full seven years. Denmark refuses the right to benefits but will give residence and work permits to immigrants if they find a job within six months. Belgium, Finland and Netherlands will only allow those with work permits to right to jobs. Sweden will apply benefit restrictions.
What has been the UK position?
Until now it had been that all citizens of the new EU member states would be treated as any Belgian, Spaniard, Italian or indeed Briton would be.
Has it now changed?
Yes. A summit between Tony Blair and a group of senior ministers in Downing Street last week agreed a package of measures to counter suggestions there could be a "flood" of people moving to the UK to claim benefits and use public services such as hospitals.
Those plans were set out on Monday and include curbs on benefits and a work registration scheme.
Why is the UK acting so late?
Ministers say action is only needed now because a number of other EU states have only recently implemented restrictions, thus leaving UK and Ireland isolated. Ministers have argued that they had been considering changes to the benefits system for some time.
The Tories say there has been a Cabinet split on the issue and point out that it has been known for a long time that the new member states would join the EU on 1 May, so it should not have come as a surprise to the UK government.
What is happening about the right to work?
Migrants from the new member states will have to get a work registration certificate from their employers, rather than having to apply for work permits before they arrive in Britain.
Ministers say will the move help them monitor whether the numbers are getting too big and will pave the way for identity cards, which will eventually go to all EU nationals living in the UK.
The home secretary said all EU citizens could travel to Britain in any case and allowing people to work openly would stop them being pushed into the black market economy.
What about benefits?
David Blunkett says people can come and work legally but not claim benefits until at least 2006.
But the details of the measures mean those working and paying tax continuously for more than a year will be able to claim benefits like income support or jobseeker's allowance if they lose their job.
And those working can also immediately claim: emergency NHS treatment; council tax benefit; housing benefit; homelessness assistance, child benefit; working tax credit; child tax credit and state pension credit.
What about those who do not want to work?
"You are not welcome" is the government's message to so-called benefits tourists. Prime Minister Tony Blair said: "If they can't support themselves, they will be put out of the country."
What's happened when other states joined?
When Portugal and Spain joined in the 1980s there were fears in France and some other countries of an influx of people, but it did not materialise and the "transitional" measures were dropped.