By Ross Hawkins
BBC political correspondent
The 'points based' system works out which professionals the UK needs
How many people should be allowed to live in Britain?
It is a simple question - but one the government has been reluctant to answer. Until now.
The new immigration minister Phil Woolas says he will not allow the population to reach 70m. That is an eye-catching promise.
Tony Blair once told MPs his government had no population policy - and agreed with their suggestion the issue was "political dynamite".
The official figures say there are over 60m people living in the UK. The population is growing at the fastest rate since the early 1960s, driven by the number of people coming to live in the country.
Nurses and ballet dancers
But some worry that if unemployment rises, the economy might struggle to cope with the new arrivals.
The Home Office believes it already has the tools it needs to reduce immigration.
Its "points-based" system works out which professionals the UK needs. Vets, nurses and ballet dancers are included in the current list.
Then it lets them into the country depending on factors such as their age, salary and qualifications.
Government officials say this will bring down the number of economic migrants allowed to enter the country, and the number allowed to stay permanently.
But Britain cannot keep out EU citizens. They have the right to work here. Nor can it turn away legitimate asylum seekers.
The Conservatives want to know what the government will do to put its new population policy into practice.
It is not clear how the Home Office could guarantee to keep the population to a certain level if it could not turn away certain categories of immigrants.
Last month the Labour MP Frank Field and the Conservative Nicholas Soames said it was time to break the link between those who take jobs in Britain, and those who stay permanently.
Many foreign workers from outside the EU would be allowed to stay for only four years.
The government rejected their ideas, the then immigration minister Liam Byrne dismissing what he called "made-up quotas".
This morning Mr Field said he had since met Home Secretary Jacqui Smith to discuss his report.
The former social security minister is once again proving influential.