The retailer will replace its existing financial services arm, M&S Money, with a full M&S Bank offering in partnership with HSBC.
Fifty in-store branches are set to roll out in the coming two years, beginning next month with a flagship branch at its Marble Arch store.
M&S Money is currently operated in M&S stores by HSBC, which bought the white-label financial service in 2004 after it was launched by M&S in 1985 to capitalise on the trust in its brand.
M&S Money already offers credit cards, but not current accounts, and these are notoriously difficult to set up, as evidenced by Tesco's much-delayed entry into the market. It currently has three million active credit card customers.
The retailer claims the bank, to be led by chief executive Colin Kersley, will offer customers private meeting areas, self service banking points, as well as a bureau de change.
It said the aim is to create a "relaxed, modern banking environment" for customers, with "personal touches" such as M&S furniture and fresh flowers.
Marketing for the brand will be led by current M&S Money head of marketing, Amanda Newman.
Marc Bolland, chief executive at Marks & Spencer said: "M&S is one of the most trusted brands on the UK high street and we've achieved this by continually listening and responding to the needs of our 21m customers.
This bank will be built on M&S values; putting the customer at the heart of the proposition and delivering the exceptional service that sets us apart from the competition."
Joe Garner, head of HSBC in the UK, said: "This is our most significant innovation in retail banking since we launched First Direct more than 22 years ago.
It is concrete evidence of HSBC increasing innovation, competition and investment in the UK."