The Council for the Protection of Rural England (CPRE) is to launch a legacy-focused campaign for the first time in two years.
Supporters of the group will receive a message urging them to leave a legacy contribution to protect the countryside from housing development plans in the UK.
An information pack created by direct marketing agency Target Direct is to be sent to supporters and members of the CPRE.
Signed by Sir Max Hastings, president of the CPRE and former editor of London's Evening Standard, the mailing claims an area of countryside the size of Bristol is lost through urban development each year. It also claims the UK countryside is threatened by the need for an additional 3.8 million homes over the next 25 years.
"Encouraging legacy giving is the major tenet of the campaign," said Stephen White, assistant director of fundraising at the CPRE. "Our message focuses on the need to protect the countryside as a legacy for future generations."