PORTLAND (AP) – The ripple effects of Bank of America Corp.’s planned acquisition of FleetBoston Financial Corp. are already being felt at Banknorth Group.

When the $47 billion merger is completed, Banknorth will become the largest banking company headquartered in New England. To capitalize on that, William J. Ryan, Banknorth’s chairman and chief executive officer, met Monday with his company’s advertising agency.

“It’s an event that happens once in our lifetimes, and it’s a very unique niche that we have,” Ryan said he told the advertising team. “No one else can say, ‘We’re the largest independent bank in New England.’ Let’s take advantage of that.”

Banknorth stockholders are also feeling the effect of the FleetBoston deal.

Stock analysts upgraded their ratings of Banknorth and investors pushed up share prices by more than 5 percent Monday on speculation that the merger makes Banknorth one of the most attractive takeover targets in the banking industry.

FleetBoston is now the largest New England-based bank, and the third largest bank in Maine with 42 branches and 500 employees. When Fleet becomes part of Bank of America, its new home will be Charlotte, N.C., where Bank of America has its headquarters, and its name will disappear.

Banknorth is the nation’s 35th-biggest bank, with assets of $25.7 billion.

Even at that size, though, Banknorth is a purely regional company without a far-flung, national branch network. Its banking subsidiaries include Peoples Heritage in Maine, Bank of New Hampshire, Banknorth Massachusetts, Banknorth Vermont, Banknorth Connecticut and Evergreen Bank in upstate New York.

If Bank of America’s purchase of FleetBoston clears regulatory hurdles, Ryan said he expects Banknorth to gain existing FleetBoston customers who encounter problems because of the merger, or who simply want to continue banking with a New England-based company.

“Even if they do it right, mistakes will be made” in the merger, he said. “In the conversion, they’re going to have to match up all their Fleet customers (with their new accounts). It’s a huge problem for customers.”

Ryan said he wouldn’t be surprised to see Banknorth’s rate for creating new accounts to increase by as much as 50 percent if customers become disenchanted with the new mega-bank.

“You just don’t know if those people have the same feeling for New England as they have for Charlotte,” he said. Banknorth, he said, is “a New England company that thinks like New England.”

Chris Buonafede, an analyst with Fox-Pitt Kelton, said he expects Banknorth to pick up new customers throughout New England. He has upgraded his firm’s rating on Banknorth to outperform, meaning he expects the stock price to do better than the market as a whole. At least two other investment firms made similar moves.

“Bank of America’s integration of Fleet is likely to be messy and will undoubtedly result in significant customer defections to other regional banks,” including Banknorth, Buonafede wrote in explaining his upgrade to investors.

While Ryan admitted that the merger means Banknorth might be seen as a possible acquisition for larger national banks, he said the company will continue to buy other regional banks. Banknorth has acquired a number of small banks in the past few years.

“We’ll be even more aggressive at going out and buying other banks,” he said. said.