Action is needed to help reverse the decline of many of Britain's best-loved woodland flowers, including the primrose, the nation's most extensive ecological woodland survey has found.

The study, Long term ecological change in British woodland (1971-2001), found that the number of plant species in 1648 specific plots in 103 native woods across England, Wales, and Scotland had declined by more than a third since they were first surveyed in 1971. Characteristic woodland plants like yellow archangel and sanicle fared worst, with 56 out of the 72 species becoming significantly less common.

Fifteen species of tree and shrub also showed a decline in numbers, along with a general fall in tree seedlings, though holly bucked the trend by spreading abundantly in many woods.

Biodiversity and Forestry Minister Jim Knight said the report did not seem to indicate a single cause for the decline in woodland flowers.

Instead, causes include:

* Woods becoming more shady due to ageing trees and inadequate woodland management; * Increasing levels of nutrients in woodland soils due to atmospheric pollution and agricultural fertilisers, possibly accentuated by less acidic soils; * The effects of climate change, with each species responding differently.

The survey shows that soils are recovering from the impact of acid rain, which was such a concern in the 1980s. Results also provided evidence that grazing pressure from deer had increased in lowland woods.

Mr Knight said that a number of government policies and programmes were already addressing the problems behind the decline.

"The Government's new policy for ancient woodland in England will help to address the decline by promoting sensitive management of our native and ancient woodlands to prevent problems like overshading," he said.

"Measures like creating buffer strips on farmland around woods, or adding to the woodland area could help to reduce the spread of nutrients into the wood from adjacent farmland and increase the habitat available for woodland species.

"However, while some plants may benefit from opening up woods, it could also enable some weedy species such as nettle and cleavers to become abundant - so careful, balanced management is essential."

Other agri-environment schemes and Forestry Commission initiatives, including reducing non-native trees and controlling livestock grazing in woodland, will help to reverse the decline in woodland wildflowers.

Mr Knight said partnerships like the Deer Initiative, a government-funded independent body that helps set up and develop local deer management groups and educates people about managing wild deer populations, were also achieving significant improvements woodland habitats.

The study was jointly commissioned by Defra, English Nature, the Countryside Council for Wales, Scottish Natural Heritage, the Forestry Commission, the Woodland Trust and the Joint Nature Conservation Committee and conducted by the Centre for Ecology and Hydrology.

In the North East work is already under way to help improve woodland habitats through the North East Community Forests Tree Nursery and the Community Forest Woodland Wildflower Project.

The tree nursery, near Sedgefield, is gearing up to meet the growing demand for locally grown trees and supplies trees directly to the Great North Forest and The Tees Forests and has the capacity to supply other organisations. A key feature of the nursery is its use as a training facility and venue for special needs young people and adults. This year it will produce more than 250,000 trees and has recently supplied 50,000 trees to Durham County Council for the Mineral Valleys Project.

The woodland wildflower project ran from 2001 to 2004 as the result of a partnership between England's 12 community forests, the Esmee Fairburn Foundation, The Countryside Agency, Local Heritage Initiative, Nationwide Building Society, with Landlife at the National Wildflower Centre and thousands of volunteers.

The aim of the project was to develop an effective strategy for the introduction of appropriate wildflowers into our woodlands and involve as many local people with projects to enhance and care for local woodlands. In the North East, staff from The Tees Forest and Great North Forest community forests worked with local communities and volunteers to collect seeds from more than 20 donor woodland sites, which there then either sown into recently planted woodland areas or grown for planting out as wildflower plugs.