Gordon Brown will intervene in the critical issue of immigration, using a major speech today 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.

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

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.

Alan Johnson, the home secretary, admitted it had taken the government a "long time" to get on top of the issue of immigration.

"It took us a long time to actually get the system in place that we've got now, which is the points based system to deal with some of the abuses ... and also to tackle the issue of how we turn round asylum seekers," he told GMTV.

"It's right that we should have a system that grants asylum to genuine asylum seekers but the process took too long."

He said timescales and backlogs had been reduced during the last 12 years from 35 months to process and a 60,000 backlog to eight months and a 6,000 backlog, but added: "I think you are right that it took a long time to get on top of this and in the time that was happening there was a huge influx in asylum seekers."

Johnson denied there had ever been an "open door policy" and said immigration had helped institutions such as the NHS in the past when medical staff from abroad were needed.

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.

In an interview in today's Daily Mail, the prime minister 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 today, 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 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.

• Alan Travis, home affairs editor
• guardian.co.uk, Monday 2 November 2009 17.14 GMT

Alan Johnson said the government 'struggled to contain the huge surge' of people arriving in the UK from conflict zones. Photograph: Dan Kitwood/Getty Images

The home secretary, Alan Johnson, has admitted the government has made mistakes in the way it has handled immigration and counter-terrorism.

Johnson said that the backlog of unreturned foreign prisoners and unresolved asylum applications were ignored for far too long and that some communities have had legitimate concerns about the strain on jobs and services. He said Labour had "struggled to contain the huge surge" in people fleeing conflict zones in the past decade.

The home secretary also conceded that some of the counter-terrorism proposals made after the 7 July 2005 bombings were "too draconian" and "not the right way to go".

Johnson, making his first major speech on immigration, at the Royal Society of Arts, insisted however that Labour had not pursued an "open-door" policy and had been the first government to introduce a system which tracked who had arrived in the country and who had left.

The home secretary also made a strong defence of the use of surveillance powers, control orders and the Prevent programme to tackle violent extremism. He strongly hinted that the government's proposals, which are expected shortly, to reform the use of Ripa (Regulation of Investigatory Powers Act) surveillance powers will stop local authorities using them for "trivial" reasons such as spying on people putting their rubbish out on the wrong day or letting their dogs foul the streets.

The home secretary said any rational debate on immigration had to recognise that there were communities disproportionately affected by immigration, where people had legitimate concerns about the strain that the growth in the local population had placed on jobs and services.

It also had to recognise that the immigration problems faced by Britain were not unique and that it was reasonable to expect that new migrants should learn the language, obey the laws and pay their taxes.

"Whilst I accept that governments of both persuasions, including this one, have been maladroit in their handling of this issue, I do believe that the UK is now far more successful at tackling immigration than most of its European and North American neighbours," he said, citing policies including the introduction of biometric visas, ID cards for foreign nationals and dealing with new asylum applications within six months, including returning failed applicants.

He admitted that Labour's record "is not perfect". "When we came to government in 1997, there was no magic button we could push immediately to resolve all the historic political and operation problems associated with immigration," he said.

"The legacy problems with unreturned foreign national prisoners and asylum seekers may have accumulated under previous administrations, but they continued to be ignored for far too long on our watch."

He said that as in other countries, the Labour government in Britain "struggled to contain the huge surge in migration – legal and illegal – that emerged from conflicts such as Kosovo, Iraq, Zimbabwe, Sri Lanka and Somalia. But this shouldn't overshadow the real and rapid progress that has been made."

E-borders, which tracks the movements of everyone in and out of the country, should be fully operational by 2014 and the completion of the backlog of legacy cases in immigration and asylum will be cleared by 2011.

He argued that the Conservative claim that Labour operated an open-door policy on immigration was a wilful misrepresentation of the facts and was a hangover from their "dog-whistle politics" of the last election.

In response to a question on counter-terrorism, Johnson said that the Labour government had probably tried to go too far after the 7 July London bombings: "That probably was an understandable feeling that we should be doing more and being more draconian was not the right way to go."