AS Prime Minister-in-waiting, Gordon Brown is due to hit the ground running this week with an ambitious and masterfully crafted programme intended to wrongfoot his Conservative rivals by winning the hearts and minds of newly eco-friendly Britain. His blueprint for the creation of five new "carbon-neutral" towns and 100,000 new, affordable homes largely lit and heated by solar panels and wind-power schemes will please the ecologically inclined, force David Cameron into a potential stand-off with his own party's not-in-my-back-yard local councils in the Tory heartlands of the south-east, and underpin his own green credentials both within the UK and in the wider global constituency.

But the devil, as always, is in the detail. His outline proposals call for the use of predominantly existing brownfield sites for building the low-emission centres of urban excellence, though he identifies only one, a disused former Ministry of Defence base in Cambridgeshire. The others, he says, will be located in response to the degree of council enthusiasm for the plan. He also claims that 40 local authorities across the UK have already expressed an interest. As a sweetener, the houses will be exempt from stamp duty and will come with road and rail links, plus purpose-built schools and health centres. He denies he intends "concreting over the countryside".

What he has not said is how it is to be paid for. It is not the first time such a project has been envisaged. Planners in Leicester spent 12 months researching the creation of a zero-carbon-emission town before abandoning the scheme earlier this year. The reason was, predictably, economic. Construction costs for houses fitted with solar panels and the other requisite technology proved to be 25% higher than normal-build.

No-one could work out how to avoid that premium, since local government lacked the cash to subsidise purchases and Whitehall would not underwrite it as a showcase development. This raises the question of how the Chancellor's "Brown Houses" will now be funded, given that one of the major reasons for pursuing the scheme is the provision of affordable homes for key workers such as medical personnel and teachers, who are increasingly unable to take the first steps on the property ladder at current salary levels. In this case, the elephant in the room may be the fact that interested councils have to bid for the chance, possibly tying those bids to some form of Private Finance Initiative.

That said, the plan is tactically brilliant. With the Tories four points ahead of Labour in the latest national YouGov poll, and Conservative control established over councils across the affluent consumer belt of the south-east, Mr Cameron is left with little wriggle-room. He will be forced either to back his party's grassroots councils, mostly fundamentally opposed to greenbelt erosion through further houses, or to support Mr Brown's initiative and face the wrath of the Tory faithful. Either way, he risks undermining his own standing as opposition leader.