Portman says it expects to make nearly 500 staff redundant as a result of the merger with Nationwide.
The society will close its administration offices based in Wolverhampton, and says it is likely that the 250 staff who work there will lose their jobs.
The society also anticipates that there will be more redundancies in its Bournemouth offices.
Despite there being 1,100 people working there at present it expects this will fall to 850.
Portman has revealed that it is in the process of distributing approximately 1.2 million merger booklet packs in advance of the Portman annual general meeting, which is to be held on April 23 2007.
Following the announcement, on September 12 2006, of the proposal to merge Nationwide and Portman, the boards of the two societies are now able to communicate further details.
The Portman merger booklet pack will cover, AGM resolutions and voting, proposed merger process and rationale, bonus details for qualifying Portman members and information for Portman savers and borrowers.
Robert Sharpe, chief executive of Portman, says: “Merging two of the largest and most successful building societies will create a mutual of impressive strength and size in the UK retail mortgage and savings market.
I believe this is a great deal for Portman members. They will enjoy an ongoing interest in the enlarged society and will benefit from a greatly increased branch network spanning the country.
They will also have access to a wider range of attractively priced products, together with more comprehensive telephone and internet banking services.
Qualifying members will receive a pre-tax merger bonus of between 200 and 1,000, if they are a saver, and 200 if they are a borrower.”
Graham Beale, chief executive designate of Nationwide, says: “We believe that Nationwide has a great deal to offer the members of Portman.
The enlarged society, with its strong commitment to mutuality, will be in an even stronger position to deliver real value to its members through better product pricing and excellent service.
As the UK’s largest mutual we continue to provide a compelling alternative to the big retail banks.”