Bank of Boston Corp. and Shawmut National Corp. will postpone a decision on a merger until January, after both banks announce earnings, said a source close to the negotiations.

Several details remain to be ironed out, the source said, who refused to speculate on the likelihood of the bank boards' approving a union.

The two banks are believed to have been seeking a merger pact for six months, though neither will officially confirm the talks. The combined institution would result in New England's largest banking company, with $55 billion of assets.

The delay was reported in The Wall Street Journal Thursday.

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