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The UK could be home to nearly three-quarters-of-a-million illegal immigrants according to research seen by ´óÏó´«Ã½ Panorama.

A previous Home Office commissioned estimate put the 2001 population at 430,000. But, according to a new calculation by the London School of Economics, the number of illegal or "irregular" immigrants and their dependents in the UK in 2007 is around 725,000.

The new figure comes from a report into the economic impact of an amnesty for illegal immigrants, commissioned by the Mayor of London.

Boris Johnson first raised the idea of granting legal status to those who have been in the UK for a considerable period of time during the 2008 mayoral election. It is a pressing issue for the capital as two-thirds of illegal immigrants are resident in London.

Tonight's Panorama on ´óÏó´«Ã½ One at 8.30pm explores the economic and social impacts of implementing such an amnesty across the UK.

Speaking to Panorama reporter Raphael Rowe, Johnson explains that, whilst he supports the deportation of illegal immigrants, he is considering the idea of an amnesty because the "catastrophic failure of the Government to control our borders" has left a legacy of more illegal immigrants than we can realistically deport.

The Government is now tightening up border controls, introducing the e-borders system which counts people in and out of the country. But, at current rates of deportation, it would take 34 years and cost nearly £8billion to clear the backlog of people who are currently in the UK illegally.

Johnson admits to Rowe it's a "hard political argument to win" but argues that we need to be practical.

"What I am trying to get people to recognise is that there are limits to what the policy to expulsions is able to achieve at the moment. Failing that, and it is failing, we need to think of a better alternative."

"If people are going to be here and we've chronically failed to kick them out it's morally right that they should contribute in their taxes to the rest of society," as a regularisation of their status would allow.

But Johnson's idea is not supported by the Conservative party or the Government. Immigration Minister, Phil Woolas, told Rowe that such a policy would encourage more illegal immigration both in terms of people trafficking and over-stayers.

"If you had an amnesty or even the impression of an amnesty, what unfortunately would happen is that people traffickers and others would see that as a pull factor to get people to the United Kingdom illegally and we would end up with a bigger problem not just for our society but for the people themselves."

The LSE report also estimates that if a floor of five years' residence were applied, 450,000 people could qualify for such regularisation. In tonight's programme Rowe meets some of these potential beneficiaries.

People like Farhan Zakaria, a 28-year-old teacher from London. Zakaria came to the UK as a 16-year-old when his father took up a job at the High Commission for Bangladesh. But the family were unaware that their right to stay in this country ended eight years ago when Mr Zakaria Senior left that job.

Up until September 2008 Zakaria was working as a Bengali and French teacher in an East London school, and paying taxes. When the authorities found out the family had lost their right to be here, he was forced to leave his job and the family are facing imminent deportation.

Zakaria said they did not realise they had now become illegal immigrants: "I put my hand up that we made a mistake and we seek for compassion. If there is such a law of compassion. I think this should be applicable to this, our situation."

Rowe also reveals how many illegal economic migrants exploit weaknesses in the UK's data systems, finding work under their real names, paying tax and national insurance on legitimate codes and even registering to vote.

"Bulwant" (not his real name) is a 36-year-old who came to the UK from India eight years ago on a visitor's visa looking for work. He lives in the Midlands with his partner, who is also here illegally, and they have a two-year-old daughter. He told Panorama that using his own passport with its expired visa he managed to get a driving licence and works under his own name driving a bus.

Midlands Factory worker "Jagir" (not his real name) also came from India, nine years ago. The 34-year-old showed our reporter that he has a bank account, a medical card, national insurance number and is registered to vote – all in his own name, despite being here illegally and so not allowed to work.

Notes to Editors

In 2005 the Home Office released an estimate of the number of illegal immigrants in the UK, putting the central estimate at 430,000, with a lower limit of 310,000 and an upper limit of 570,000. www.homeoffice.gov.uk/rds/pdfs05/rdsolr2905.pdf

The 2005 Home Office commissioned estimate was based on 2001 census data and is a reflection of that population at 2001.

Whilst the 2007 figure updates the 2001 estimate, the 2001 figure did not include British-born children of illegal immigrants. The LSE's 2007 figure includes an estimate of British-born children, for both 2001 and 2007.

The LSE report gives a central estimate of 725,000 with a lower limit of 524,000 and an upper limit of 947,000.

The LSE report put the London population of illegal immigrants at 518,000 with lower and upper estimates of 345,000 and 721,000.

The LSE report estimates that if a five-year residence requirement were applied, 450,000 people would be eligible for regularisation with a lower estimate of 325,000 and an upper estimate of 590,000.

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