The Court of Appeal has allowed a challenge to a £500,000 legacy to continue, even though the deceased woman left clear instructions that her estate should be split between three animal charities and exclude her estranged daughter.
But the court ruled this week that Ilott had the option to continue her claim in the High Court against her mother's estate on the basis that she was an "adult child" in financial need.
Melita Jackson's letter said that if her daughter brought a claim against her estate she wanted her executors to "defend such a claim, as I can see no reason why my daughter should benefit in any way from my estate bearing in mind the distress and worry she has caused me over the years".
It added that Jackson had seen her daughter only twice since she left home at the age of 17 in 1978.
The charities expressed dismay at the ruling and said it could open the floodgates for legal challenges from aggrieved relatives.
Kim Hamilton, chief executive of the Blue Cross, said the judgement – which overturned a previous Court of Appeal decision – could have a "devastating impact on charities at a time when the government is trying to boost charitable giving to help us respond to an increased demand for our services".
James Aspden, a partner at Wilsons Solicitors who is representing the three charities, said: "The Court of Appeal has reinterpreted 30 years of law and left in its place a lack of clear guidance, which creates further uncertainty about a person's right to leave money to people or organisations of their choice."