Tony Blair has insisted that closer
co-operation with Europe over asylum and immigration will not mean losing control of Britain's borders.
Mr Blair defended the changes
The prime minister said the UK would only participate in EU-wide action where it was in Britain's interests.
The Tories say the change surrenders Britain's national veto and question how an opt-out would work in practice.
But Mr Blair told his monthly news conference Britain had the "best of both worlds" and would keep the veto.
"There is no question of Britain giving up our veto on our border
controls," he said.
"With the Treaty of Amsterdam seven years ago, we secured the absolute right to opt in to any of the asylum and immigration provisions that we wanted to in Europe.
"Unless we opt in we are not affected by it. And what this actually gives us
is the best of both worlds.
"We are not obliged to have any of the European rules here.
"But where we
decide in a particular area, for example to halt the trafficking in people, for
example to make sure that there are proper restrictions on some of the European borders that end up affecting our country, it allows us to opt in and take part in these measures."
Home Secretary David Blunkett met his EU counterparts in Luxembourg on Monday to consider proposals aimed at streamlining decision making on the issue.
The 25 member-states are expected to scrap the requirement for unanimous agreement on immigration policy, in favour of the qualified majority voting
Under this scheme larger states such as Britain are expected to have more power than the smaller EU states.
Britain is expected to retain an "opt-in" right which will allow it to ignore any measures it disagrees with.
Conservative leader Michael Howard said the prime minister should stop "duping" the public and tell the truth.
"Yet again he is all talk," he said. "The Charter of Fundamental Rights, which he has signed up to, will give the final say to unelected judges.
"That means Britain will lose control of its borders."
But Liberal Democrat spokesman Mark Oaten called the change pragmatic and argued it gave a better chance of producing a European asylum solution.
"If we don't work together it means some countries can ignore their
responsibilities at the expense of their neighbours," said Mr Oaten.
UK Independence Party MEP Nigel Farage complained: "Our Westminster Parliament is being reduced to county council status within the European Union, with control over immigration, home affairs, defence and foreign policy all set to be ceded to Brussels."
Best of both worlds
The Home Office says signing up to qualified majority voting on immigration and asylum is helps Britain, which can drive forward policy rather than go at the pace of the slowest EU member.
Mr Blunkett has already said he will not support plans for a common asylum system like the scheme being floated in Brussels.
Monday's meeting approved what is known as the Hague programme of justice and home affairs policies, which will replace a five-year plan agreed by European ministers in 1999.
The programme includes a plan for common deportation procedures and for spreading those granted refugee status more evenly across the EU.
It is expected to be approved by the heads of government, including Tony Blair, at a European Council summit in Brussels on 5 November.