The merger creates a combined society of over 3.5 million members, 224 branches and assets of £33bn.

It follows approval by N&P members at a Special General Meeting in August and FSA approval in September.

The N&P name will be retained as a separate and distinct brand within YBS, the UK’s second largest building society.

Iain Cornish, chief executive of YBS, said: “The enlarged Yorkshire will continue to be owned and run for the benefit of its members. We will be working to integrate the societies and make sure there is a seamless transition. Members using N&P branches will see the same name over the door and the same staff they know and value.”

A spokesperson for N&P said: “All customers can be reassured that Yorkshire is an efficient and financially strong mutual organisation and is focused on delivering long term good value products and excellent service to its members.

“YBS has the financial strength and depth to provide you with a stronger and more secure society long-term. In the current uncertain and volatile economic climate, that’s reassuring to know.”

Membership of the society will date from whenever customers became members of N&P and savers who now have over the FSCS limit because of the merger will be sent letters to advise them on moving their savings.

N&P faced significant problems earlier this year when it was found it advised some 3,200 people to invest in Keydata products, a company which subsequently went into administration.

It was investigated by the FSA and fined £1.4million for failing to take reasonable care to ensure the suitability of its advice.