Image caption The UK Border Agency removed 20,000 illegal workers in 2009

In his small two-bedroom flat, Felipe looks through a carefully filed thick pile of payslips.

They show he has been making regular tax and national insurance contributions and have been issued by a large well-known company, where he has been working for six years. Yet Felipe is an illegal immigrant with no right to work in the United Kingdom.

Professor Ian Gordon, the co-author of a report on the issue of illegal working, says that he thinks Felipe's story is far from unusual.

Prof Gordon, of the London School of Economics, told File on 4 that "probably two-thirds" of illegal workers are paying "some kind of taxes and national insurance".

"I don't think that it's the case that the great majority of irregular migrants live in and work in some entirely cut-off economy," he adds.

Forged documents

"I think that most of them are on the margins to start with and then well inside the margins of the mainstream economy with some kind of paperwork to support them."

Some estimate that Felipe is one of about 200,000 illegal workers in London, although there could be more than that.

The UK Border Agency in the South East carries out regular raids on employers. In the last financial year in the region, there were 22,714 immigration removals.

Back in his flat, Felipe produces his P60 - his annual tax statement. It shows he paid about £750 in national insurance over the course of a year.

I'm doing the job that most English people wouldn't do. I think I have never seen an English person cleaning a toilet Felipe, Illegal worker from Colombia

Felipe, who is from Colombia and has been living in the UK illegally for nine years, claims he bought his national insurance number from a fraudster.

"I just use it as anyone else as anyone else would use his national insurance. I just hand it to my boss and fill out the forms and that's it," he says.

He adds: "I see it [paying tax and national insurance] as a kind of payback for me being here. And I don't mind - and I prefer that - as long as I can work I am happy.

"The reason for me being here is not just because I want to become rich or because I want to come and take your jobs.

"I'm doing the job that most English people wouldn't do. I think I have never seen an English person cleaning a toilet."

With tears in his eyes, he says he is doing it for his two young children and regularly sends money back to them in Colombia.

"I don't want to harm anyone…. It's hard to be judged as a criminal when all you are doing is what you think is best for your family. What I'm doing cleaning toilets - is that a crime?"

Bogus National Insurance

For Andrew Large, chief executive of the Cleaning and Support Services Association which represents contract cleaning companies, this is a familiar story.

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"This is a problem that my members have come across on a number of occasions. It seems quite clearly that people are able to use national insurance numbers to which they have no right," he says.

"They may be legitimate national insurance numbers that have been misappropriated or alternatively, they may be wholly fictitious."

Mr Large says the government needs to take responsibility.

"From our perspective, it would seem that inside Her Majesty's Revenue and Customs… there doesn't seem to be an effective process for handling any anomalies with regard to the number and the name which might emerge as part of this process. They certainly do not appear to be communicating with the UK Border Agency."

A government statement said it was committed to stopping illegal working but added: "It is an employer's responsibility to ensure they check their employee's paperwork to ensure they have the right to work in the UK.

"If they can't provide this proof, they will be liable for a fine of up to £10,000 per illegal worker.

"All government departments are working together to clamp down on illegal workers and the companies that employ them."

File on 4 is broadcast on BBC Radio 4 on Tuesday, 6 July 2010, at 2000 BST, repeated Sunday, 11 July, at 1700 BST. You can listen via BBC iPlayer or download the podcast.