NEW YORK (Deutsche Presse-Agentur) -- Warren Buffett, the world's third richest man, said Tuesday that he has prostate cancer and will undergo radiation treatment.

"The good news is that I've been told by my doctors that my condition is not remotely life-threatening or even debilitating in any meaningful way," the 81-year-old investor wrote to shareholders. His message was released by his company, Berkshire-Hathaway.

Buffett said he would start two months of radiation therapy in July.

The diagnosis of stage-one prostate cancer was made last week after a blood test showed a jump in the so-called PSA level. That prompted the decision for a biopsy, followed by a CAT scan, bone scan and, on Tuesday, an MRI, Buffett said.

Buffett wrote that the tests showed no incidence of cancer "anywhere else in my body."

"I feel great -- as if I were in my normal excellent health -- and my energy level is 100 per cent," Buffett wrote.

The limelight has been on Buffett recently during the heated debate in Washington over increasing the tax rate on high earners. Buffett, known for his down-to-earth approach and lack of pretense, has angered much of the politically conservative business community by siding with US President Barack Obama and advocating for higher taxes.

Buffett frequently mentions the unfairness of a system that taxes his secretary at a higher rate than him, since he benefits from corporate and individual tax reductions mostly pushed through under former president George W Bush.

In another philosophical twist typical of the investor, Buffett conceded his eventual mortality in his cancer message.

"I will let shareholders know immediately should my health situation change," Buffett said. "Eventually, of course, it will; but I believe that day is a long way off."

According to Forbes magazine, Buffett's 44-billion-dollar fortune makes him the world's third richest man. His world-wide fans admire not only his investing acumen, which has earned him the nickname "Oracle of Oklahoma," but also his moderate life style. His occasional weaknesses are steak and Coca Cola.

Buffett took over the small textile company, Berkshire Hathaway, half a century ago, turning it into one of the most highly valued companies in the world. Its market valuation is 200 billion dollars.

Berkshire's investment programme includes large companies such as Coca-Cola and IBM and its own subsidiaries ranging from fast food to insurance. Buffett and his 88-year-old partner Charlie Munger have given no indication they'll take their hand off the tiller anytime soon.

But Buffett has indicated that he knows who will succeed him when he gives up the reins. The name however is kept under lock and key at headquarters. He has not even informed the person in question, but only said that his successor will be one of the managers of his subsidiary companies.

In 2006, Buffett said he would start donating most of his fortune to charity. He planned to give most of the money to the charity founded by Microsoft's Bill Gates, which was already the richest philanthropic organization in the world.