The change to her will, made after she died in a car crash in Paris, will ensure that the butler, Paul Burrell, whom she described as her "rock", is among the beneficiaries of her pounds 21m estate.
The posthumous amendment will also mean that her sons, Princes William and Harry, can inherit their share before they are 25.
The original will was made in June 1993 and was not changed when she and the Prince of Wales divorced three years later. She had left her estate to be divided between her sons equally, with no account taken of the fact that Prince William will inherit the throne.
Last December, however, solicitors acting for the executors and Princes William and Harry obtained a court order to vary the will, and a posthumous amendment was added to the original document to reflect the Princess's love for her godchildren.
A spokeswoman for the Law Society said posthumous changes to a will were not uncommon. "It happens quite often and as long as all the beneficiaries agree with the changes then it can be done," she explained.
"It sometimes happens when, for example, the children of someone who has died want to give something to someone who was very close to the deceased but who was not mentioned in the will."
It is thought that three-quarters of the Princess's estate will go to her sons. Prince William, 15, and Prince Harry, 13, will also receive the stake in Spencer House in London, which their mother shared with her sisters Lady Sarah McCorquodale and Lady Jane Fellowes, her brother Earl Spencer, and a group of trustees.
The 18th century house, which overlooks Green Park, is now a museum and art gallery, and is also used as a banqueting suite.
Each one of her godchildren, aged between two and 16, will be able to choose a personal item which had belonged to the Princess.
Her godchildren come from a variety of backgrounds and include the children of friends such as Dominic Lawson and his wife Rosa Monckton and royalty such as Prince Philippos, the 11-year-old son of ex-King Constantine of Greece.
Mr Burrell is expected to receive pounds 50,000 made at the bequest of the Spencer family.
The bulk of her estate comes from the Princess's divorce settlement, with the remainder from investments and her personal fortune.
The will will be available from the Principal Registry of the Family Division at Somerset House in London for 75p - or for pounds 2 by post - and demand is expected to be huge.
The Princess's most senior aide, Michael Gibbens, said yesterday that her sons would be "very distressed" by an attack on the Princess by the rock star Noel Gallagher.
In an interview with a local paper in Australia, where Oasis are on tour, Gallagher said: "So she died in a car crash, big deal."
Using a flurry of obscenities when referring to the national outpouring of grief, the Oasis star is reported to have said: "Fat ****ing British housewives are a pathetic bunch of ****ers, do you know what I mean?
"Half the people there probably wouldn't visit their grandmother's grave ... then they go and throw flowers at some coffin of some bird they've never met because she'd done some work for charity.
"I wasn't interested in anything she'd done for charity - or her personal life. So she died in a car crash. Big ****ing deal. It didn't affect me. I really couldn't give a s**t at the end of the day."
Mr Gibbens said: "This sort of statement can only be incredibly distressing to members of her family and particularly to the two boys.
"I expect to be approached by many devastated people over this. It's appalling."Reuse content