Members of the Portman building society have voted overwhelmingly in favour of a merger with Nationwide.
The Portman name is to disappear
The tie-up, which was backed by more than 90% of Portman shareholders, will create a lender with assets worth more than £150m and 13 million members.
Portman had promised its members a windfall payment of between £200 and £1,000 should they back the deal.
Nationwide is currently the UK's largest building society, while Bournemouth-based Portman is the third.
The deal is expected to be completed by September, and the new company will take the Nationwide name.
According to the two firms, the merger will result in a number of job cuts as the Portman's 153 branches and Nationwide's 860 outlets are combined and overlapping operations are trimmed.
Nationwide has more than 16,000 staff, and Portman currently has about 2,500 staff.
The Nationwide's head office in Swindon will become the new firm's headquarters.
When the deal was announced, Portman said its main offices in Bournemouth would be a key business centre within the enlarged society and would remain a significant employer in that region.