The information requested falls within the responsibility of the UK Statistics Authority. I have asked the Authority to reply.
Letter from Karen Dunnell, dated December 2008:
As National Statistician I have been asked to reply to your question, pursuant to the answer of 18th November 2008 (Official Report, column 408W) on immigration, regarding what the level of net immigration is at which the population of the UK would remain below 70 million on unchanged assumptions about birth rates, mortality and emigration levels on the basis of the most recent population projections. (243465)
Migration assumptions for national population projections are conventionally expressed in terms of net migration (immigration less emigration) rather than for the gross flows separately. However, nominal immigration and emigration totals are used in the projection process mainly to enable plausible age distributions to be calculated for the assumed net migration totals.
The 2006-based principal (or central) projection assumed a long-term annual net inflow to the UK of +190,000 persons a year. In this projection, the population of the UK is projected to reach 70 million in 2028. To answer your question, further population projections have been produced by reducing the nominal immigration totals applied in the 2006-based principal projection, but leaving the fertility, mortality and emigration assumptions unchanged. Based on these assumptions, with a long-term net migration inflow of +60,000 a year, the population would reach 70 million in the 2070s. However, with a net migration inflow of +50,000 a year the population would be just below 70 million in 2081, although it would still be continuing to rise.
In practice, this may not be a realistic scenario. If immigration fell permanently to levels well below those experienced in recent years, it is likely that this would, in time, lead to a reduction in the level of emigration as well, it is possible to have a range of combinations of levels of immigration and emigration consistent with any particular level of net migration and these would lead to different projected population sizes.