An NAO report says Natural England is well on the way to meeting a government target to ensure that 95 per cent of all English SSSIs are in a healthy or improving condition by 2010. By March this year 83 per cent of SSSI land area - some 889,000ha - was in target condition.
However, the report adds that Natural England's administrative and oversight functions must be improved. All SSSIs are supposed to be assessed every six years but Natural England is falling behind on this, it warns.
NAO head Tim Burr said: "Extra resources have gone into SSSIs and results are starting to show. But many will take time to regenerate fully, so a sustained management effort and clear conservation objectives are needed for long-term success."
Natural England chief executive Helen Phillips said: "After much hard work and investment, SSSIs are in a better state than they have been for years. However, all too often they remain isolated pockets of rich wildlife in a wider countryside where biodiversity is diminishing.
"To guarantee long-term survival of our wildlife and build in resilience to climate change, we need to tackle the environmental health of entire landscapes, focusing on wide restoration of habitats and ecosystems."
Natural England's Role in Improving SSSIs is available at PlanningResource.co.uk/doc.