In short, Professor Rowthorn's review of current research does not find any significant boosts to the economy from mass immigration. This means that the other negative effects of mass immigration, such as building on the green belt or crowding in the cities, means that the consequences of mass immigration ‘are mostly negative for the existing population of the UK and their descendants’.
From the Daily Mail report:
Professor Rowthorn, a former consultant to the International Monetary Fund and the UN Trade and Development Commission, said in his report that there may be no economic gains from immigration at all.
If there are, they will be outweighed by extra costs imposed by the strain on housing, land, schools, hospitals, water supplies and transport.
He said in a landmark report for the Civitas think tank: ‘Unrestrained population growth would eventually have a negative impact on the standard of living through its environmental effects such as overcrowding, congestion and loss of amenity.
The Professor notes that it is possible for mass immigration to have an overall negative effect:
‘If many of the immigrants fail to get jobs, or if they end up in low skill jobs or displace native workers, large-scale immigration will have a negative impact on GDP per capita and on government finances,’ he added.
The impact could be positive or negative but either way it is unlikely to be very large. The only thing that is certain is that immigration on the present scale, if it continues, will lead to much faster population growth and a much larger total GDP than would otherwise be the case, with consequent pressure on infrastructure and the environment.’
(Related to this last paragraph see the data on the disability pension in the UK here.)