It will be years before the true impact of Sure Start on the lives of children is known, but the initial indications are that it will be among New Labour's social policy success stories. Designed to provide a range of social and health support to deprived families with very young children, it has been generously funded, has got through to some traditionally hard-to-reach families, and is often highly regarded by those who use the service. It has a reputation among users, not to mention a budget, that local authority services can only dream of - and sometimes do.

But Sure Start is about to lose its cherished independence. Most of its 524 local programmes will be "mainstreamed" within local authorities next year, although programmes in 21 council areas will go over this April. They will become part of the new network of local authority-run children's centres. In his spending review last summer chancellor Gordon Brown promised 2,500 of these by 2008 before being trumped by the prime minister, who pledged 3,500 by 2010.

It has led to a heated debate between Norman Glass, Sure Start's creator, who claims the programme is being abolished in all but name, and Naomi Eisenstadt, director of the Sure Start Unit, who argues that the underlying principles will be retained despite the changes. The evident uncertainties about Sure Start's future are shared by many of its local programme managers.

For many local managers the local authority takeover brings opportunities and threats. It is an opportunity to forge close alliances with the emerging children's trusts and to be part of the Every Child Matters agenda. But there are also worries about council bureaucracy and priorities which have little to do with the intensive preventive work that Sure Start is able to provide to families.

"On balance the local authority takeover is a threat," says John Fowler, who manages the Sheerness Sure Start, which is part of the first round of transfers taking place this April. He had an initial meeting with Kent Council in early February, but he still has no idea how much money he will have or what services he will be able to provide.

"I know nothing about my budget and I've got five weeks to go," Fowler says. "The council isn't prevaricating, it's desperately trying to get its act together. It's very complicated for them too - they don't know the situation because the government hasn't let them know. And we're right at the butt end of all this."

It is an anxious time not only for him, but for his staff too. "My staff are saying, hang on, if we might be losing the money, hadn't we better move on? If things remain unsettled and uncertain, I risk losing a team it has taken three years to build up."

Angela Graham, head of children's policy and performance in Kent social services, admits that the council is still in discussion with the government, but says that it may simply "passport" the money directly to the localities this year and mainstream them when the children's centres start work next year. She doesn't know what the funding will be; however, tough decisions lie ahead.

"Children's centres will not be as generously funded as Sure Start has been, so there will be a shortfall," she says. "We'll preserve what works and discard what doesn't - and funding will be a factor in that."

What Sure Start programmes most fear losing is their localised, community-led spirit. Parents have had a major influence over how money and resources are used, but councils may be wary of giving so much power to communities. For example, social services departments may want to target families that programmes had previously overlooked.

Lynda Hassall, manager of Carlisle South Sure Start, thinks that the programme's preventive focus could be sidelined by the pressure of statutory work. "Child protection is rightly seen to have the priority in local authorities and is usually at the expense of prevention," she says. "Parental participation is crucial - one of the tenets on which the whole Sure Start programme is based is that it should be community-led with professional input. What really worries me about local authorities is the lack of thinking about involving parents."

Fowler shares these concerns, pointing to cultural resistances in both health and social services. "They are quite nervous about community-led approaches. It's about ceding power to non-professionals - there's a great deal of professional jealously, a great distrust of the people."

Not surprisingly Naomi Eisenstadt displays more optimism. Performance targets and inspection frameworks will help ensure that the local autonomy of programmes is maintained, she says, and that parental involvement is more than tokenistic. She admits that resources will be spread "slightly more thinly" as Sure Start extends beyond its contentious geographical and age boundaries, but the old model lacked inclusivity and was no longer sustainable.

"I'm proud of what we've achieved," Eisenstadt says, "but if we really believe in children's trusts then we have to take some risks. Why should councils interfere if the service is working at neighbourhood level?

"There's an arrogance in saying that only we can do it and nobody else can. The thing that really depresses me is that all of a sudden we are trying to defend an institution and forgetting about the outcomes for children. The point of Sure Start was to break out of institutional silos."

Tensions in the relationship with child protection were plain to see in the national evaluation of Sure Start published in January. In a survey of social services professionals, the difficulty of reconciling the demands of child protection and prevention was "by far and away" the most important for them. Statutory responsibility for child protection exerted a powerful influence on social services involvement in Sure Start. As the evaluation puts it: "In the most pessimistic cases this was seen as an entrenched problem, which against their every best effort was almost impossible to resolve."

Angela Graham agrees that protection and prevention will be hard to marry, despite years of "refocusing" that was designed to shift social services resources from firefighting crises to early intervention. She says that Kent is still thinking through the relationship between child protection services and Sure Start, adding that the council is "trying very hard" not to consider switching resources to the former. But nor are resources likely to move the other way, although she expects pressure from local programmes to make up the shortfall in funding.

"One of the problems is that we're not quite working with the same kinds of families," Graham says. "For example, they're not working particularly well with the hard-to-reach families who come into the child protection system.

"They have done what local communities and parents want them to do, but the sorts of families we work with are not the families who join in with their local communities in running Sure Start. I hope that we'll be able to line up our approaches more effectively."

But Fowler thinks that the writing is on the wall for the clearly demarcated deprived communities that have reaped the benefits of Sure Start until now. "These communities have been depressed for so long, they jolly well deserve to be spoilt. The glory days are over."