Some 40 volunteers, who work for both the marine and terrestrial environments, attended a reception in the garden of 10 Downing Street.
The Prime Minister told the group: "I am grateful for the contribution that people right across the country make every day to their local environmental and wildlife projects, giving up their time without pay.
“The inspirational work they do helps create a better world for us all. It is my pleasure to welcome these volunteers to No 10, so I can say thank you in person."
Jan Loveridge, a volunteer for Cornwall Wildlife Trust who co-ordinates the Marine Strandings Network, later said: “It was really good to know that he and the Government are aware of the contribution volunteers make to monitoring and caring for the environment and wildlife.
“Volunteering is a fantastic way to get involved in vital, scientific research and it was a privilege to represent my fellow volunteers who work so hard to protect the marine environment and its wildlife.
“We're really fortunate that Cornwall Wildlife Trust recognises that 'voluntary' doesn't mean 'amateur' and they make the most of the skills and experience that volunteers have to offer.
“We have volunteers in the Marine Strandings Network from all walks of life, including experts in cetaceans, seals, birds and basking sharks. The only difference between us and many others working in marine research is that we contribute our knowledge and experience for free to help fight for a cause we believe in.”
Danny Cooper, a volunteer for The Isles of Scilly Wildlife Trust who works in its Wildlife Information Centre, appreciated Brown’s address and was pleased to be able to “meet volunteers from all over the country and find out about other people’s experiences and the type of work they carry out for their local Wildlife Trusts”.
The Wildlife Trusts - www.wildlifetrusts.org