After years of consideration and planning, Waterbury’s Webster Bank will open a substantial branch office in Boston’s financial district Monday, according to a bank spokeswoman.

Webster’s Boston office will function as a sort of “mini-corporate headquarters,” and hold a full-service retail branch with most of the bank’s lines of business, including business and middle-market banking, spokeswoman Brenda Greene told The Commercial Record.

Webster, which already has 20 branches in Massachusetts, has been considering a downtown Boston move for several years, Greene said. The bank will hold a ribbon-cutting ceremony at the new location Dec. 9.

The expansion comes despite Webster’s poor performance in recent quarters stemming from loan-related losses. Webster reported a $19.3 million loss in the third quarter this year, compared to a $16.5 million loss for the same period in 2008. In the first nine months of 2009, the bank lost $61.9 million.

In July, the bank announced that it was changing its charter to allow equity investment firm Warburg Pincus to invest $115 million in Webster in exchange for 15.2 percent of all common stock outstanding and preferred shares. The deal was completed in October.

As part of the agreement, David A. Coulter, a Warburg Pincus managing director who co-leads the firm’s financial services investment activities, will join Webster’s board. Coulter is the former chairman and CEO of BankAmerica Corp. and vice chairman of JP Morgan Chase.