Turner chaired the Financial Services Authority during the global financial crisis, when taxpayers had to bail out several UK lenders.

"As we broaden our offering to customers with a wider range of business banking products, Adair's experience across financial regulation will be incredibly useful in ensuring we continue to scale robustly," Rishi Khosla, CEO and co-founder of OakNorth, said in a statement.

Launched in 2015, OakNorth's current chairman, Cyrus Ardalan, steps down at the end of the year after eight years in the role, in accordance with Britain's Corporate Governance Code, the company said.

"The business has impressive growth plans, as well as ambitious plans for sustainability - notably to be net zero for all its emissions by 2035, which will involve a huge amount of work with its customers," Turner said.

(Reporting by Huw Jones; Editing by Hugh Lawson and David Goodman)