The big growth in oil extracted from shale rock means the US will not need to import any crude within two decades, the former boss of BP has said.

Lord Browne told a conference in Oxford the US would be "completely independent of imported oil, probably by 2030".

He also said the amount of shale gas in the US was "effectively infinite".

Shale oil and gas is extracted using a method called fracking, but the process has been controversial because of the environmental risks associated with it.

Lord Browne is a director of fracking firm Cuadrilla.

He told the Resource 2012 forum on water, food and energy scarcity that the development of shale oil and gas was "quite extraordinary", and that the world was now entering the "latest age of primary energy".