Anthony Thomson, who previously co-founded Metro Bank (>> Metro Bank PLC), one of the most successful challengers to Britain's big four lenders, will be succeeded by Bridget Rosewell, chair of Atom Bank's audit committee.

Since opening for business in 2014 as a digital-only bank focused on savings accounts and mortgages, Atom has collected more than 1 billion pounds ($1.43 billion) in deposits and lent the same amount to small businesses and homeowners.

It is one of a number of branch-free banks that have sprung up in Britain in recent years to take on the likes of Barclays (>> Barclays), HSBC (>> HSBC Holdings), Royal Bank of Scotland (>> Royal Bank of Scotland Group) and Lloyds (>> Lloyds Banking Group).

Atom has gone from "being a big idea disguised as a small bank to a big idea and a big bank", Thomson said in a statement.

"I have loved doing this with Atom but, as the bank moves into a new phase of development, I feel the time is right for me to step down from the role of chairman."

While attracting less limelight than some of its peers, such as app-only bank Monzo, Atom has quietly raised significant funding from high-profile backers including Spanish lender BBVA (>> Banco Bilbao Vizcaya Argentaria).

So far it has received 250 million pounds from investors and is planning a further fundraising round.

(Reporting by Emma Rumney; Editing by David Goodman)