Mr Prescott has taken only a small, faltering step to help our cities

This government is not short of large ambitions. It wants to abolish child poverty at home and abroad, save the planet and win two consecutive elections. Now it has added to its "to do" list - reversing the post-war migration out of Britain's big cities. Yesterday's white paper represents an attempt at social engineering on a grand scale, and its likely chances of success need to be assessed against that background.

This government is not short of large ambitions. It wants to abolish child poverty at home and abroad, save the planet and win two consecutive elections. Now it has added to its "to do" list - reversing the post-war migration out of Britain's big cities. Yesterday's white paper represents an attempt at social engineering on a grand scale, and its likely chances of success need to be assessed against that background.

The reasons for the movement of population, the reversal of the 19th-century move to the cities, are fundamental. People want gardens and good schools, and the motor car makes it possible to live in villages, market towns or deep suburbia and still work in urban jobs. Electronic communications accelerate the exodus.

Thus the population of northern and Midlands cities will continue to decline, while the "exurbs" - beyond the suburbs - will continue to grow. Over the next 25 years, the populations of Cambridgeshire, Dorset, Bedfordshire and West Sussex, up to 150 miles from the London hub, are expected grow by up to a quarter.

The ambition of John Prescott's white paper, to act as caring midwife to that unconvincing cliché, an "urban renaissance", is laudable enough, although it is hard to see the measures proposed making much difference to the desirability of living in the centres of cities by 2025.

Indeed, the few measures clothed in the grandiloquence of a Command paper fall well short of a decisive tilting of the economic incentives acting on people's decisions about where they live and work.

On the nub issue of encouraging the development of new homes on "brownfield" rather than "greenfield" sites, the Government has ducked the tough choice. A central problem is the fact that VAT is charged on renovating existing buildings but not on building new ones. Imposing VAT on new homes would sharply increase the price of starter homes, and would in effect be a tax on the floating, aspirational voter. But cutting VAT on renovation worsens the anomaly and is limited by European Union rules. So Gordon Brown has chosen to fiddle around the edges.

The Chancellor also ducked the challenge posed by the environmental impact fees proposed by Lord Rogers, the architect of the Millennium Dome. This is an attempt to put a price on the value to the community as a whole of unspoilt green spaces. That attempt is right in principle, although horribly difficult in practice. The impression given by the white paper, however, is that this Government has not even grappled with the problem, because it is more susceptible to short-term electoral pressures than long-term ecological ones.

There is a contradiction, moreover, with another white paper on policies for the countryside, expected in the next few weeks. Yesterday's aims to make living in the inner cities more attractive; the next white paper aims to make living in the countryside more attractive. Reversing population movements requires sticks as well as carrots, but New Labour prefers to offer happy pills all round.

There may be a role for government in nurturing a vibrant culture of good city design, integrating policies for public transport, public spaces and - above all - better schools, to produce a high-density lifestyle that people would choose over the more isolated attractions of the countryside. But Mr Prescott's white paper is only a stuttering, small step towards it.