In letters to officials and published today on his Twitter account, King Abdullah's nephew warned that it was alarming that "92 per cent of the government budget relies on oil".

"The world's reliance on OPEC oil, especially the production of Saudi Arabia, is in a clear and continuous drop," he wrote in letter addressed to oil minister Ali al-Naimi.

He said the threat from shale gas is "definitely coming", pointing to progress in that sector in North America and Australia.

"Revenue diversification is a must, and that necessitates a clear vision that should be implemented immediately," the Arab world's richest businessman said.

He proposed creating a sovereign wealth fund to manage the kingdom's reserves, which he said stood at USD 676 billion in March, citing official figures.

Alwaleed also urged action to press ahead with plans to develop nuclear and renewable energy to "reduce local consumption of oil as soon as possible".

Alwaleed was 26th on the Forbes 2013 list of billionaires, with an estimated wealth of USD 20 billion -- but he challenged the rating, saying he was worth USD 9.6 billion more.