Billionaire Bill Gates says another financial crash “is a certainty”.

The Microsoft founder answered a broad range of questions in a Reddit AMA this week, including one from a user concerned that global finance system would see a repeat of the 2008 GFC.

Mr Gates said he does not know when, but that another financial crash “is a certainty”.

“Fortunately we got through that one [the 2008 financial crash] reasonably well,” he said.

“Despite this prediction of bumps ahead, I am quite optimistic about how innovation and capitalism will improve the situation for humans everywhere.”

Economists have been more clear about the concerns.

Steve Keen - head of economics, politics and history at the UK's Kingston University – wrote last year in The Conversation that “mainstream economists need to get their heads out of the sand”.

He said personal debt would likely be the cause the next time around.

Professor Keen says banks risk generating too much credit for the wealth generated by economic growth to handles.

He said issues in China, “whose credit bubble is easily the fastest growing in the history of capitalism”, were already creating concern.

“Indian economist Amit Kapoor sees history repeating, saying; ‘China is on a path eerily similar to the pre-crisis US’” Professor Keen wrote.