The society's board said market conditions had forced them to link up with the UK's largest building society. The loss is for the half year up to June.

The Derbyshire has its headquarters in Duffield and a branch in King Street, Belper.

Customers will notice few changes at first, with branches and the society's headquarters remaining. There would be no immediate job losses or branches closing, the Derbyshire has said. Customers have been told their money is safe.

Jon Smart, from the Derbyshire, said: "Nationwide want to keep the branch network intact.

"It will carry out an operational review once the deal is rubber stamped to prevent doubling up, but customers will probably notice very few changes – the branches will remain and look the same.

"Nationwide will retain the headquarters at Duffield Hall as an operational base."

Graham Picken, chief executive of the Derbyshire, said: "I am pleased that the Derbyshire has come to an agreement with Nationwide that assures the future for our savers and borrowers.

"The board recognised a number of financial uncertainties facing the Derbyshire, and has taken this prudent and pre-emptive action in seeking a solution to secure the interests of our members over the long-term.

"I have every confidence that the Derbyshire members will benefit from being part of the enlarged society."

The Derbyshire has assets of £7.1 billion, 50 branches and 485,000 members. The Cheshire Building Society is also merging with the Nationwide in a similar move.

Graham Beale, Nationwide's chief executive, said: "The Derbyshire and the Cheshire have independently concluded that a merger with Nationwide is in the best interests of their savers and borrowers given the financial issues faced by both societies.

"The core member businesses of both societies are in good shape and have a better future as part of a larger organisation."