Page last updated at 15:37 GMT, Thursday, 30 June 2005 16:37 UK

'Working out' illegal immigration

By Daniel Sandford
BBC Home Affairs correspondent

A group of illegal immigrants hide their faces as they try to enter the UK
The "best guess" for the number of illegal migrants is 430,000

The government has revealed figures on the number of illegal immigrants living in the UK. But what do they really mean?

Few things are more sensitive in politics than immigration.

Furthermore, you might imagine that a key part of immigration control would be having an idea of how many foreign nationals are living in the UK without authorisation - illegal immigrants, in other words.

Not least because for 10 years people smuggling, black market working and false asylum claims have been fundamental issues in British political life.

That is why it is so surprising that until now the Home Office has made no effort to estimate the number of illegal immigrants in the UK.

It is obviously a flawed figure

Perhaps they did not want to know, as they were worried what the answer might be.

Well, we do now finally have an estimate of between 310,000 and 570,000, with 430,000 as the "best guess" for the number of unauthorised migrants in the UK.

It is obviously a flawed figure. The range is very wide, and it is based on the 2001 census returns.

Work it out

In simple terms, the Home Office statisticians have subtracted the number of foreign nationals in the census from the known number of legal foreign migrants.

They have obviously had to build in a large amount of leeway for the high number of illegal immigrants who are unlikely to fill out a census form.

Cynics might say it is helpful in winning the argument for ID cards
Is this really a good way of working it out?

Well, the Home Office researchers say a surprising number of illegal immigrants do have to engage with society, because they need access to schools and healthcare, so the census is not as unreliable as might be thought.

Anyway, however flawed, it is the best method they have got.

So why publish the research now? Well, cynics might say it is helpful in winning the argument for ID cards.

Illegal working and immigration have been key factors quoted in favour of the cards

Why not publish it before then? Well, there was the small matter of an election in which immigration was a crucial issue. During the campaign the prime minister famously insisted in a Newsnight interview there was no way of knowing how many illegal immigrants there are.

In response to both those theories the straight-laced statisticians at the Home Office - who like to think they are above the political fray - say quite simply that the research is being published now because this is when it is ready.

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