In a speech in London this week he said: "The incumbents risk becoming merely capital-providing utilities that operate in a highly regulated, less profitable environment, a situation unlikely to be tolerated by shareholders."

Jenkins says a series of Uber-style disruptions in the industry could shrink headcount at traditional big banks by as much as 50%, while profitability in some areas could collapse by over 60% — huge predictions from a man who, until recently, ran one of Britain's biggest banks.

He adds: "In my view only a few [incumbent banks] will have the courage and decisiveness to win in this new field."

The problem? Financial technology, better known as fintech.

Jenkins, who was ousted as Barclays CEO in July, says a new wave of tech-savvy startups that can do things better, faster, and cheaper than the big banks will disrupt their traditional businesses like lending, payments, and wealth management.