Gordon Brown will intervene in the critical issue of immigration, using a major speech tomorrow to promise that migrant workers will only be used to fill jobs temporarily in parts of the economy where there are labour shortages.

He will make it a requirement that government-sanctioned training schemes are created to ensure that unskilled British workers can ultimately take on the jobs in sectors where there are genuine skills shortages, such as catering, supply teaching or some skilled medical and engineering jobs.

The speech will be seen as an effort to give meaning to his promise of "British jobs for British workers". His intervention follows private polling conducted during the summer by the Unite trade union showing that immigration is the single biggest issue leading natural Labour voters to defect either to the more extreme parties, such as the British National party, or into refusing to vote at all.

Ministers have acknowledged that they have ceded ground to the BNP either by not talking about immigration or by not confronting the BNP's true politics.

The issue is likely to become more potent as unemployment increases and the Conservatives claim the number of migrants in the UK is the result of a deliberate government strategy to create a multicultural Britain.

Brown has not made a significant speech on immigration since he became prime minister and tomorrow's speech is seen by some ministers close to the issue as belated, if welcome.

In an interview in tomorrow's Daily Mail, he insisted that immigration had always been a source of "economic, social and cultural strength to Britain", but added: "I understand people's concerns when they hear suggestions that levels of immigration are going to rise. Especially in difficult economic circumstances, people have concerns … They want to be assured that the system is tough and fair. They want to be assured that newcomers to the country will accept their responsibilities … obey all the laws,speaking English is important, making a contribution."

In his speech tomorrow, the prime minister will again reject Tory proposals for an annual cap on immigrants, arguing that the policy is unworkable and cannot be implemented due to the free movement of workers inside the EU.

Ministers also claim the flexibility inherent in the government's points system introduced in 2008 allows the government to raise or lower the bar on who can be allowed into the UK, in effect having the same impact as an annual quota.

Brown will also propose a tightening of the "labour market test" that allows employers to recruit migrants from outside the settled workforce for a skilled job only if they can show no suitably qualified settled worker can fill the job.

Under the test, a job vacancy must also be advertised for two weeks before a migrant can be recruited. The prime minister will say that in future the job will have to be advertised for a month. Brown will also highlight the government's decision to require employers to set up accredited skills training schemes in any areas of the economy where there is a shortage of skills requiring employers to recruit from abroad.

In an effort to take the heat out of the argument, he will say there has been a 44% fall in net immigration over the last year, and as a result of the points system the number of people who can enter Britain for work without skills has been reduced. He added in the Daily Mail interview: "There is a new determination to train people in the skills that we need. We want to ensure … that we don't have to bring to the country people with skills when we can develop those skills quickly."

The Migration Advisory Committee, a government advisory body, said in a report last week that the number of people in the government's skills shortage list had fallen in a year from 700,000 to 500,000. That represented less than 2% of all employees. It also found that net immigration for work-related reasons has fallen throughout 2008.