"It's a big book. It's got value for our members," said Trevor Bayley, finance director of the Britannia, declining to disclose how much the society had paid for the mortgages.
He said it was one of the society's largest deals and did not rule out further acquisitions. "If we see a transaction which looks like good value we would consider it."
However, he noted, there were few other mortgage portfolios of this size for sale on the market.
Citibank customers will not notice immediately any difference to the way their mortgages are handled.
Britannia intends to write to the 30,000 Citibank customers whose mortgages it now owns in the next few weeks to inform them how their mortgages will be administered.
The mortgages cover a range of properties throughout the country and the loans range from variable rate to special loans for customers with small deposits and fixed-rate deals.
Citibank Customer Bank is left with pounds 500m of mortgages in its portfolio, which it intends to keep.
The mortgages it sold to Britannia were for customers who only used Citibank for their mortgage and no other service.
"This transfer brings our UK mortgage business into line with our global strategy," said Meredith Williams, UK executive director of Citibank Consumer Bank.Reuse content