The new points-based system was announced in 2006
UK firms will have to prove they cannot find skilled workers from the European Economic Area before looking elsewhere for immigrants, ministers say.
Outlining plans, the Home Office said bosses would not be able to fill posts before advertising first in the UK.
The points-based guidelines for skilled workers also say most will need a job offer before coming to the country.
Immigration Minister Liam Byrne said the new system would mean UK workers "get a fair crack of the whip".
Points for skilled workers, such as teachers, nurses and engineers, will be awarded according to qualifications and salary prospects.
Only those who work in an area where there is a shortage of qualified people will be allowed into the UK without a job offer.
The government says the rules would have reduced skilled migrants last year from outside the EEA (the EU plus Norway, Iceland and Liechtenstein) by 12%.
Immigration Minister Liam Byrne said: "Our new points system means that British job seekers get the first crack of the whip and that only the skilled migrants we actually need will be able to come."
The Home Office says that in the 12 months to last September, 65,000 skilled workers from outside the EEA were allowed in, but under the new rules there would have been almost 8,000 fewer.
But Sir Andrew Green, chairman of Migrationwatch UK, said: "The government's claim that their new points-based system will result in lower levels of immigration is pure spin.
"The scheme has no limits and, in fact, will probably result in even higher levels of immigration."
The Home Office says workers such as sportsmen and entertainers, attending one-off events, such as the Edinburgh International Festival, will not require points, but will need a visitors' visa.
The new immigration points system, which is based on education, previous salary and age, was announced two years ago and is being phased in this year.
The more skills a worker has, and the more those skills are in demand, the more points they will gain, increasing the likelihood of entry.
The first stage, applying to highly skilled workers, came into force in February.
It already lets most young graduates work in the UK if they have earned a salary equivalent in their country to £40,000.
An independent committee will advise ministers on which skills the economy needs.
The Home Office says that there is no need for any unskilled workers from outside the European Economic Area.