A legal fight is brewing over the surprise $15 billion merger announced Friday morning between Wachovia Corp. and Wells Fargo & Co. that upsets a Citigroup Inc.-Wachovia transaction announced on Monday.

In an interview, Wells Fargo chief executive John Stumpf said Charlotte-based Wachovia and San Francisco-based Wells have a merger agreement and that “we’re very confident that this will lead to a combination.” Wells said it expects the deal to be completed by year end.

Citigroup, however, issued a statement demanding Wachovia and Wells Fargo terminate the transaction. Citi said the deal was a clear breach of an exclusivity agreement between Citi and Wachovia and that it “has substantial legal rights regarding Wachovia and this transaction.”

On Monday, New York-based Citi announced that it was buying most of Wachovia’s banking operations and assets for $2.16 billion in stock, with assistance from the Federal Deposit Insurance Corp. That deal would have left behind a stand-alone company housing brokerage and asset management businesses, but its future has been uncertain.