The four-year scheme has been unveiled by Yorkshire Water and will involve more than a dozen sites, beginning at Timble Ings, near Harrogate.
Many of the sites fall within water catchment areas for the company’s reservoirs and the work will help to protect raw water quality, using “back-to-nature” solutions rather than the firm having to invest in expensive technology.
Thousands of non-native trees planted in the 19th and 20th centuries will be removed through traditional forestry techniques including the use of horses and carts to take away the timber. Native species such as oak and alder and shrubs including hazel and hawthorn will then be planted.
It is hoped wildlife habitats will be boosted at a time when experts agree biodiversity is in decline across Europe.
Yorkshire Water’s recreation and catchment manager, Geoff Lomas, said: “Ancient woodlands are essentially the UK equivalent to the Amazon rainforests and it’s hard to stress just how valuable they are both in terms of our heritage and from a biodiversity point of view.
“Once an ancient woodland is lost, it’s gone forever, which is why we’re taking a lead and investing such a significant amount to save these precious sites and enhance them by planting native species which will help to create ecologically diverse habits that are home to an incredible range of plants, insects and animals.”
While ancient woodlands date back to at least the 17th century, many of the historic areas in Yorkshire are several thousand years old.
The Forestry Commission, Natural England and local conservation experts are also involved in the project.