Oxfam is launching a new telemarketing drive to encourage existing supporters to opt in to the Government's Gift Aid scheme.
The decision follows a successful trial last year that raised an extra £1 million for the charity.
"Gift Aid is a great way for our supporters to ensure that their donations stretch as far as possible," said Pierino Cioffi, Oxfam appeals officer.
"Telemarketing has proved the ideal way to talk to those supporters."
The campaign will start in early April with mail packs being sent out to supporters. The packs will familiarise donors with Gift Aid and explain how the scheme helps charities by allowing them to recover 28p for every £1 donated by tax payers.
Calls will then be made to around 20,000 supporters who have not responded to previous direct mail activity on Gift Aid.
"We were very pleased with the results of the trial," said Martin Williams, marketing director of call centre business The Listening Company, which is co-ordinating the telemarketing drive. "We haven't set targets for what we expect to raise this time because each supporter gives a different amount, but we are positive about the results that can be achieved."
Last year's drive proved to be cost effective with £16.70 being returned for every £1 spent on the telemarketing campaign. Of the supporters called, 25 per cent gave verbal consent or promised to return a Gift Aid consent form.