Peter Steer, a partner in the charities department of Wilsons solicitors in Salisbury, said: "Those who leave money to charities in a will can often be without family. They are quite vulnerable and people can take advantage of this.
"Those who are elderly or ill, particularly those with dementia, may be approached by people coming to them with offers that are not in their interests."
Steer's comments come after the Institute of Chartered Accountants' decision last week to strike off an accountant who took £141,000 from an elderly client who had left her estate to Dorothy House Hospice Care and Age Concern England.
Gervase O'Donovan, a former deputy mayor of Bath, claims his client had agreed to lend him the money and allowed him to make payments from her account using power of attorney.