The Tories have said Labour is floundering on immigration
Employers will have to advertise jobs to UK workers for at least a month before they can be offered overseas, the government has announced.
Home Secretary Alan Johnson said that from next year the legal minimum period to post vacancies on Jobcentre Plus would be doubled from two weeks.
He added that this meant non-EU workers could not be used as a "cheaper alternative" to those from the UK.
But the Tories said Labour was "overselling" the changes.
The number of overseas-born workers in the UK reached 3.8 million last year.
In a speech, Mr Johnson said the government had accepted all the recommendations made recently by its Migration Advisory Committee.
He announced the minimum salary to allow someone from overseas to qualify as a skilled worker in the UK would rise from £17,000 to £20,000.
The minimum qualification period for non-EU workers transferring to a company's UK base would increase from six months to a year, Mr Johnson added.
The Home Office estimates that one in 10 of the non-EU workers given permits last year would be excluded under the new rules.
Mr Johnson said: "These changes will ensure that businesses can recruit the skilled foreign workers that the economy needs, but not at the expense of British workers, nor as a cheaper alternative to investing in the skills of the existing workforce."
The UK introduced a points-based immigration system earlier this year.
Under this, employers can only recruit "skilled" workers from non-EU countries if they cannot fill a vacancy or the occupation is on a list of shortage occupations.
The Conservatives have called for an annual cap on non-EU migrants.
Shadow immigration minister Damian Green said: "The home secretary should stop overselling what are pretty minor changes. The idea that this would cut work permits by 10% is just a fantasy figure.
"The way to control work permits is to have an annual limit, which the government short-sightedly refuses to introduce."