By Dominic Casciani
BBC News community affairs
You don't need a degree in advanced mathematics to know the Home Office's formula to count illegal immigrants is political dynamite.
No past estimates of illegal immigrants have been published
But as government announcements go, the calculations on Britain's undeclared population (see the factbox for the explanation) was as surprising as they come.
Until Thursday, every time someone asked the Home Office for a number, it said they couldn't be counted.
But now we have an official estimate, there are some tough questions to be asked of the government's immigration policy.
New immigration minister Tony McNulty has slipped behind his desk signalling he may take a different approach to some of his predecessors, conceding to an audience of immigration experts on Thursday that tough talk from ministers is not always helpful in aiding public understanding.
But what do we understand 570,000 illegal immigrants to mean?
Size of a city
To critics of the UK's immigration policies it equates to the population of Edinburgh. That is a stark image of the public burden were all those people to be in the same place at the same time.
On the other side of the coin, were 570,000 people spread evenly, they would represent just less than 1% of the British population.
Total foreign born-population identified in the census minus estimated total legal foreign-born, which includes, permanent, temporary and quasi-legal residents, minus those thought to have died or emigrated equals estimated illegal migrant population
Result: 310,000 to 570,000 people
In reality, experience shows migrants of all sorts initially gather where services have historically been geared towards their communities.
Take Camberwell's Walworth Road in south London, for instance. Anyone local will tell you that it's a hub of Nigerian culture. To the west, Earls Court is a home for Antipodeans. If you were a Nigerian student looking to earn some cash driving a taxi, or a New Zealand gap-year kid seeking cash-in-hand bar work, you'd know where to go - especially if your visa has run out.
Take away that gloss and the burden on public services remains a key issue in the migration debate, not least because it's difficult to measure what it actually means.
Take asylum dispersal for instance. Some GPs outside of London have complained they have not had the resources to cope on the languages front. Schools in some areas have had to learn new skills in integrating refugees. In other areas, particularly those in London used to unplanned arrivals, migrants have found it easier to integrate and get on.
What happens now?
But what ministers now do about these people is the great question. By definition, illegal entrants are the hardest to find as they may have never come into contact with the authorities.
One Chinese community leader recently told the BBC he believed there were 80,000 of his compatriots in the UK without so much as a single rubber stamp on a piece of paper, admittedly a back-of-the-envelope estimate.
SPAIN'S ILLEGAL IMMIGRANTS AMNESTIES
Source: Home Office/BBC reports
Where are they? Working in the fields and catering industry in terrible conditions, he said, trying to make cash to send home to their villages.
Identifying some of the failed but missing asylum seekers, the second category of illegal entry, should be easier if the immigration service has kept a list of their names and the addresses where it sent their rejection letters.
But visa overstayers, the third category, are probably as difficult as illegal entrants to find.
The big question however is what to do with them?
Ministers can either ignore them, try and find and deport them, or, most controversially, "regularise" their position by offering an amnesty.
Woking on the basis that most illegal entrants are in the black economy, then ignoring may appear to policy makers to be the easiest thing to do. Some economists would argue many industrialised countries have done so for decades, quietly welcoming the deflationary pressure that undocumented workers have on wages.
That approach however does nothing to prevent future illegal immigration or do anything to attack the international criminal networks often associated with the smuggling of workers across borders.
Critics of the government, principally the Conservatives, argue that had ministers got hold of the situation and controlled borders when the crisis began to unfold, then there wouldn't be a problem in the first place. Ministers retort that a national debate on migration needs to recognise how the movement of people is affecting all rich nations.
Deporting is also problematic, as the asylum experience has shown. In 2001 the Home Office ditched a pledge to remove 30,000 failed applicants a year. Today, removals run at about 3,500 a quarter.
This leaves regularisation or amnesty exercises - a route that would bring both political and practical problems.
Amnesties are common in some Mediterranean countries. Supporters say they reduce illegal working and the hold of criminal gangs. But the counter argument is they reward illegal immigration and act as a pull factor for more of the same.
Spain has had six amnesties since 1985. The results have shown an upward trend in the size of the illegal population. In other words, Spain's experience is that amnesties readily identify people for taxation, but may not influence the movements.
There has been no indication from any minister that the UK will entertain the idea of an amnesty. But was the release of yesterday's figure an act of open government, a tactic in the identity card debate - or the first move towards what could be a very explosive public debate on amnesties?