An immigration officer at Heathrow Airport
The government has launched the first stage of a new points-based system for migrants from outside the EU.
It will initially only apply to highly skilled workers already in the country who want to extend their stay.
But by the end of 2008, every graduate with good English, on £40,000 or the local equivalent, will potentially have enough points to seek work in the UK.
The Tories say there should be a yearly cap on immigration, the Lib Dems say the rules could cause skills shortages.
Under the new system, skilled workers in occupations where there is a shortage will also be able to enter, provided they have a job offer.
But low skilled workers from outside the EU will be barred for the foreseeable future. The government believes it can fill all manual work vacancies from EU countries which, with the exception of Romania and Bulgaria, face no restrictions on working in the UK.
Prime Minister Gordon Brown said people wanted a system of managed migration. He said: "If you've got a skill to offer, we're happy to look at what you have to offer to our country.
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"But obviously if we don't need the skill, if you haven't got a skill, then there's a case for us saying that this is not the right time for you to apply to come to our country."
The government says it is the biggest change in UK immigration policy "in a generation" and will attract migrants with the right skills to boost Britain's economy while easing pressure on local public services.
But the Conservatives say the changes are "over-hyped" and will not make a significant difference to the numbers entering the country.
The first phase - Tier One - replaces the existing highly skilled migrant programme, which is also based around points.
It is designed to attract entrepreneurs with significant sums to invest in British business as well as highly qualified people who the government believe will boost the economy.
All applicants will have to pass an English test - unless they have £1m or more to invest.
HAVE YOUR SAY
Immigration reforms are as good as the people who act on them
Skills and earning potential will also be taken into account - although much will depend on the country in which applicants live.
For example, someone applying for entry from a poor country, such as Nigeria or Afghanistan, will have to prove annual earnings of at least £4,000, while somebody applying from a wealthier country will have to have a previous salary of £40,000 or more.
Tier two, to be launched later this year, will focus on filling gaps in the labour market - an independent committee will advise ministers on which skills the economy needs.
Businesses who want to bring in skilled workers will need licences.
Other tiers covering temporary workers, young people and students will be introduced later.
Immigration minister Liam Byrne said the changes, which he says are based on Australia's immigration system, offer enough flexibility to respond to changing economic conditions.
"If and when we need to raise the points score that a migrant needs to come to Britain we can do that and do it instantly, rather than setting an arbitrary number," he told BBC Radio 4's Today programme.
He rejected Conservative calls for an annual cap on immigration, which he warned could create "chronic skills crises".
Mr Byrne has also said the new system will ease pressure on services and community tensions in parts of the UK experiencing high levels of immigration, even though many of the new arrivals are from Eastern European countries not covered by the points-based system.
But asked if he thought there were too many immigrants in Britain, he said "it can't be reduced to such simplicities".
Mr Byrne has also introduced new fines for those found to be employing illegal immigrants as part of a move towards more managed migration.
But shadow immigration minister Damian Green said a "sensible" policy would include a cap on the number of migrants who can come to the UK.
He said: "We've seen real strains in some areas on housing, on police, on hospitals and on school places and the new system makes no attempt to address that at all.
"You still don't know whether you're getting the right number of people that the social services, the public services, can absorb."
For the Liberal Democrats, Tom Brake said: "This is going to discriminate against people perhaps working in the care sector because they're unlikely to get a high number of points, or, for instance, people working in the restaurant trade.
"Those jobs are very unlikely to be filled by people from other EU countries and so we could then experience significant shortages in those areas."