Free Wills Fortnight, which is backed by a consortium of charities, including Age Concern England, Barnardo's and Shelter, generated more than £1m for the ten charities that signed up to the first scheme in 2005.
But the consortium is to run two fortnights this year and is already aiming to expand further in 2007.
When the scheme first ran, Stephen Maund, the managing director of Capacity Marketing - which created the scheme and the consortium - wrote to the largest 100 charities and accepted the first to reply from different cause areas.
Lindsay Boswell, chief executive of the Institute of Fundraising, said: "As the largest single source of voluntary income for UK charities, legacy fundraising has gone from strength to strength in recent years. Our latest research suggests legacy fundraising accounts for 40 per cent of voluntary income and that legacy income has increased by 36 per cent in the past three years."
Around 80,000 legacies are left to charities each year. According to Legacy Foresight, a research programme funded by charities to predict legacy income, the average legacy donation to charity is more than £40,000.
Total legacies to charity are valued at £1.5bn a year.
Under the Free Wills scheme, solicitors write wills for over-55s free of charge. Charities pay the solicitors a discounted rate for the service.
This year, the British Lung Foundation and The National Osteoporosis Society have joined the initiative. The scheme will run this year between 18 and 31 May in 15 previously untargeted towns in England and Wales, and again in September in the towns targeted last year. Radio campaigns and mailings will publicise the scheme.
"We choose ten charities for each fortnight to make it straightforward," said a spokesman for the scheme.
Rachel Brady, direct marketing manager at the British Lung Foundation, said: "Legacies are a valuable source of income for us, so our participation was a great opportunity."