WASHINGTON – Bank of America and FleetBoston Financial Corp. won approval from the Federal Reserve on Monday for a merger creating the third-largest U.S. bank, a behemoth holding nearly $1 trillion in assets and stretching from California through the South and up to New England.

The Fed’s board of governors voted 6-0 to clear the merger, determining that Bank of America’s acquisition of FleetBoston would not threaten competition or unduly concentrate banking resources. The new institution, with about 5,700 branches, would have assets trailing only Citigroup and another planned bank megamerger between Chicago-based Bank One and Wall Street powerhouse J.P. Morgan Chase.

The proposed purchase of FleetBoston – a deal estimated at $47 billion when announced last October – has been met with much anxiety in Boston, which would lose its last major hometown bank, and to a lesser extent in other communities across the country worried about diminished competition and local control.

The central bank’s approval removed the last major regulatory hurdle for the combination, which already had been blessed by the Justice Department and the Federal Trade Commission. The merger also must be approved by shareholders of both banks, allowing it to be completed early next month.

At public hearings held by the Fed in January in Boston and San Francisco, witnesses expressed concerns about predatory lending, possible closing of rural bank branches and the trend toward big banks getting bigger.

Last month a federal judge in Honolulu – saying he lacked jurisdiction – dismissed a lawsuit against the central bank brought by Native Hawaiians seeking to halt the merger. The Fed noted that Bank of America had recently affirmed its intention to complete its goal for mortgage lending to Native Hawaiians.

The banks maintain that the merger will benefit consumers by giving them an expanded choice of ATMs, branches and banking products.