The cost of the action against a souvenir manufacturer, Franklin Mint Company, is likely to draw further criticism of the fund's legal costs, which have already soaked up millions of pounds. Setting up the fund cost pounds 5m, most of itlegal fees.
The action, which is expected to last 10 months, centres on the fund's attempts to protect the intellectual property rights of the princess, and secure its own commercial activities in the process, by seeking to stop Franklin Mint from producing a range of Princess Diana merchandise, including plates, dolls and rings.
The fund claims that products bearing her image infringe her intellectual property rights.
The case will be heard in Los Angeles in March.
Franklin Mint is thought to have made at least pounds 20m with products bearing the Diana image. Its lawyer, Robert Meyer, said that it had been selling Diana products since 1981 and the princess had never tried to stop it. "No judgment that this court can bring can unring the bells of 17 years of sales," he said.
But the fund's chief executive, Dr Andrew Purkis, said the case must be fought to send a signal to others wishing to commercially exploit the image of the princess. Dr Purkis said: "The stakes are very high. But with the sort of income coming in, this sort of expenditure is justified. It is also important to send a message that we assert our rights.
"If we win, we will get the costs back as well as damages."
The importance of protecting intellectual property rights was demonstrated in the figures presented for the period from 4 September 1997 to 31 December 1998, showing that 75 per cent of the fund's total income came from commercial sales.
Dr Purkis told the briefing at the fund's County Hall headquarters in London that out of the fund's income of pounds 94m, pounds 71m had been raised from donations or royalties linked to the sale of products.
The fund was working on ways of continuing to generate this kind of "new money" for the charity in the future.
A review showed the most successful products have included the Royal Mail commemorative stamps, which brought in pounds 8.9m, and the "Princess" Beanie Babies, which provided pounds 5.8m by December 1998.
Sir Elton John's version of "Candle In The Wind" was clearly the major source of income in this area, bringing in pounds 33m from worldwide sales and publishing income.
By 31 December 1998 the fund had already committed more than pounds 16m to more than 100 organisations and Dr Purkis said this would increase by at least pounds 7m in 1999, with further grants to be announced later this year.
He said: "The fund has come a very long way in a very short time. Rarely has any charity been born in such extraordinary, turbulent circumstances.
"But with pounds 16m already committed in grants, the fund has emerged as an influential champion of charitable causes, reaching out to some of the most disadvantaged and vulnerable people in this country and overseas."