Vivienne Parry, a former trustee of the Diana Memorial Fund, described Mr Burrell's soon-to-be-published book, A Royal Duty, as a "cynical exploitation".
The serialisation of the controversial book in the Daily Mirror this week has led to a war of words between the ex-servant and Buckingham Palace, with Princes William and Harry describing it as a "cold and overt betrayal".
Ms Parry said she thought Mr Burrell's book had been "in the making for a very long time".
"I think this is the most cynical exploitation of correspondence that he never would have seen while the princess was alive," she said.
When asked whether there was any way the former butler's revelations could be halted, Ms Parry replied: "The only person that can get a grip on this is William. I think he needs to step in and sort this out and put Burrell back in his pantry where he belongs."
Her criticism came as Mr Burrell prepared for a round of media interviews to defend his book.
Mr Burrell has refused to apologise for writing his book, due to be published today, despite the princes' scathing criticism.
In an interview with a Sunday paper, Mr Burrell accused royal courtiers of poisoning the brothers' "little minds".
And he claimed the letters, allegedly to and from Diana and serialised in the Daily Mirror this week, were just the "tip of the iceberg".
"I know where the boundary is and I do not cross that line," he said. "Anything I reveal is to illustrate a fact. Other books have been rather sad betrayals."
William and Harry have said they would be prepared to meet Mr Burrell after appealing for him to end his revelations.
Diana's former private secretary Patrick Jephson also said the controversy over Mr Burrell had been a public relations "disaster" for the Royal Family.
He said they needed to go back to its "core mission" in order to avoid similar "accidents" in the future. "The Royal Family needs to do less, but it needs to do it more and it needs to do it better," he said.
Mr Jephson added that Mr Burrell's book was "totally different" to his own controversial work, Shadows Of A Princess.
"The circumstances were very different then, the issues were different," he told the BBC. "My book was a factual account of Diana's life and work in the centre of the Royal Family and it was not based on or derived from private correspondence or revealed personal details.
"It was necessary at that time to put on record facts about the Princess's life and work which other people were trying to airbrush out of history or certainly distort.
"If I had not written it, those facts would not have gone on record."