Prof says his '13,000 EU migrants' report 'misinterpreted'
The economist who predicted that opening UK borders to 10 new EU countries in 2004 would increase the population by 13,000 a year has accused MPs of misinterpreting his figures.
Politicians have said the forecast was "spectacularly wrong" and "laughable".
But Prof Christian Dustmann believes none can have read his 2003 report.
He said it made clear immigration would be much higher if, as happened, Germany and other countries decided to curb access to their labour markets.
He told the BBC that at the time he was compiling the report, which was commissioned by the Home Office, "everybody expected" that "other countries, in particular Germany, would likewise open up their labour markets".
Although some Conservative MPs voiced fears when the report was published in June 2003 that Germany would seek to impose restrictions - and called for the UK to do likewise - Germany did not confirm its plans with the European Commission until November 2003.
In the end, of the 15 prior member states only the UK, Ireland and Sweden fully opened their borders to nationals of the 10 states that joined the EU in May 2004, which included Poland, the Czech Republic, and Hungary.
The report had briefly considered what might happen if Germany did introduce transitional controls. About "one in three immigrants who had intended to migrate to work in Germany would instead migrate to the UK", it estimated.
This would increase the 13,000 figure to 46,000 a year if you extrapolated from the average figure for his predictions for German immigration, Prof Dustmann explained - although the figure is not directly stated in the report. The Office for National Statistics now estimates that the actual figure was about 50,000 a year, he added: "Not very far off."
If you extrapolate from the report's highest estimate of German immigration, the figure for net migration - the difference between people moving to the UK from the new EU's 2004 intake and the numbers moving the other way - increases to about 83,000.
But the University College London economist's report was cited by Labour Home Office Minister Beverley Hughes in June 2003 as evidence to support her claim that: "The number coming here for employment will be minimal."
And the 13,000 figure the report gave became the figure that the then Labour government allowed to stick, despite the decisions of other countries to bring in transitional controls.
Professor Dustmann regrets that his conclusions, which were full of caveats about the unreliability of the available data and the difficulty of producing accurate forecasts, have since been comprehensively "rubbished".
In 2008, the then Lib Dem home affairs spokesman Chris Huhne argued that the "scale of the error is breathtaking. Actual immigration was 1,373% higher than the forecast.
"In years of scrutinising government projections, which I have to own up to as a former economist... I cannot remember another projection, even for difficult objectives such as borrowing, that was wrong by such an order of magnitude.
"As we know, Christopher Columbus thought that he had discovered [a new route to] India, when in fact he was in America. By comparison with the Home Office, he was a practitioner of pinpoint navigation."
The Conservative shadow foreign affairs minister Lord Howell of Guildford in 2006 described the forecast as "spectacularly wrong". "Laughably," he had said, "the departments here were out by about 2,200%, which is, frankly, not a brilliant forecast".
As recently as December 2012, Conservative MP Philip Hollobone called it a "disgracefully inadequate estimate".
But professor Dustmann believes they have compounded the error on Germany by counting only the number of people arriving in the UK from the 2004 accession countries, and ignoring those who subsequently returned home.
He also notes that his predictions were supposed to be an average for 10 years from 2004, so it was too early to draw conclusions, as Lord Howell did, in 2006.
The professor concludes: "We are pretty proud of that report, it is a good piece of careful, very careful, academic work where we point out with great care all the caveats of doing any predictions under the circumstances of not very good and incomplete data.
"And we say over and over again that one has to take extreme caution in interpreting the numbers."
The economist is director of the Centre for Research and Analysis on Migration and has published numerous reports on the impact on the UK of immigration from Europe.
On the end of transitional controls on UK immigration from Bulgaria and Romania, due on 1 January 2014, he says: "All other European countries will open up their labour markets to Bulgaria and Romania, not just the UK, so we are in a very different situation that we were in 2004.
"At the same time, the traditional countries of destination for Romanians are clearly Italy, because the language is very similar and there are already big communities in Italy, and then the central European countries."
But he is coy about revealing more detailed predictions. "You won't get a number out of me!"