Washington - Forbes magazine alleged Tuesday that Saudi tycoon Prince Alwaleed bin Talal "systematically exaggerates" his wealth after the billionaire accused the magazine of understating the size of his fortune.

Alwaleed announced that he was severing his ties with Forbes after it reported in its annual billionaires' list that he was only worth US$20 billion (S$25 billion), US$9.6 billion less than the prince claims.

In its 2013 list published on Monday, Forbes ranked Alwaleed the world's 26th top billionaire, naming him the richest man in the Arab world.

But immediately after the list came out, the prince's office and his Kingdom Holding investment group announced they "have ended their long-standing relationship with the Forbes Billionaires List."

In a statement, Kingdom said that its officials would no longer work with Forbes' valuation teams, accusing Forbes of "intentional biases and inconsistencies" and "bias against Middle East investors".

"Prince Alwaleed has taken this step as he felt he could no longer participate in a process which resulted in the use of incorrect data and seemed designed to disadvantage Middle Eastern investors and institutions," it said.

But Kingdom added it would continue to cooperate with the rival billionaires list of Bloomberg, which ranks Alwaleed 16th in the world with US$28 billion.

Alwaleed, 58 and a nephew of Saudi King Abdullah, owns large stakes of Apple, Citigroup, News Corp, Facebook and Twitter as well as a luxury hotel chain through Kingdom Holding.

Forbes argued in a long article published Tuesday that it had sought to establish his wealth based on the underlying value of Kingdom's investments rather than the price of its Riyadh-traded shares.

Forbes explained that the share price has moved up sharply and inexplicably each year for the past several years just at the time Forbes researches its billionaires estimates, and the effect has been to inflate the company's market value far out of line with its intrinsic value.

It noted that Alwaleed controls 95 per cent of the shares, and few are traded actively over the Riyadh exchange.

"For the past few years former Alwaleed executives have been telling me that the prince, while indeed one of the richest men in the world, systematically exaggerates his net worth by several billion dollars," Forbes writer Kerry Dolan said.

"The value that the prince puts on his holdings at times feels like an alternate reality, including his publicly traded Kingdom Holding, which rises and falls based on factors that, coincidentally, seem more tied to the Forbes billionaires list than fundamentals."

Forbes described the prince as hounding and pressuring the magazine to use his own US$29.6 billion figure, "which would return him to the top ten position he has craved."

In the list out Monday, Liliane Bettencourt, of the French L'Oreal cosmetics and beauty empire, was ninth with US$30 billion, and her fellow luxury tycoon Bernard Arnault, leader of LVMH, was tenth with US$29 billion.

But Alwaleed and Kingdom accused Forbes of "unsupported and biased" allegations about the company's stock, and of applying "unexplained and purely arbitrary" discounts to the company's holdings.

"We have worked very openly with the Forbes team over the years and have on multiple occasions pointed out problems with their methodology that need correction," said Kingdom's chief financial officer, Shadi Sanbar, in the statement.

"However, after several years of our efforts to correct mistakes falling on deaf ears, we have decided that Forbes has no intention of improving the accuracy of their valuation."