LONDON and ABU DHABI — Prince Alwaleed bin Talal, the billionaire Saudi Arabian investor, has warned that his country’s oil-dependent economy is increasingly vulnerable to competition from the US shale revolution, setting him at odds with his country’s oil ministry and Opec officials.

In an open letter addressed to Ali Naimi, the Saudi oil minister, the prince called on the government to accelerate plans to diversify the economy.

‘Our country is facing continuous threat because of its almost total dependency on oil,