A survey by the think tank found that three-quarters of the charities that were questioned spent £200,000 a year or less on marketing legacies, compared with an average annual legacy income of £14m. This equates to a marketing budget of up to 2 per cent of their legacy income.
The research, which was commissioned by the Arthritis Research Campaign to analyse legacy marketing, was based on information from 54 charities.
"It's surprising how little money charities are putting back into their marketing activities, considering how much money the charities in our survey were getting out of them," said Joe Saxton, co-founder of nfpSynergy.
"Legacy marketing is not an area where we see huge saturation, so there is a great deal of scope to increase the level of legacy giving without conflict between charities."
Charities could invest in staff to speak to potential legators, said Saxton.
"I don't think it is a coincidence that we have seen the NSPCC appointing a senior person to look after legacies," he added. "Cancer Research UK has done the same."