Britain must set a maximum population level if it is to avoid destroying the environment and putting national security at risk, say experts.
The Optimum Population Trust has written to ministers calling for a policy of 'zero net migration' - matching numbers allowed into Britain each year to numbers leaving.
The UK's population is projected to increase from 60 to 70million over the next 20 years, and to 85million by 2081.
The trust, a panel of academics and environmentalists, says achieving zero net migration would cut Britain's population in 2081 to 57million.
Mass immigration 'feeds through into rising greenhouse gas emissions' and more congestion, the experts say.
The trust warns that because Britain can produce only 30 per cent of the food, energy and other goods that it needs, it will become increasingly vulnerable to 'resource nationalism' as foreign powers hoard their own scarce resources.