There are signs of a breakthrough in the protracted casino negotiations between Suffolk Downs and Mayor Thomas M. Menino’s administration, as the two sides have agreed on the framework of a deal that will allow the track’s casino project to move forward to a referendum vote, according to two people with knowledge of the negotiations.

Though details remain to be finalized on paper, the deal will be the state’s richest casino host-community agreement to date, with payments to Boston dwarfing the $25 million annually that a rival developer, Wynn Resorts, has agreed to pay to Everett, one of the people said.

Suffolk Downs would not confirm negotiators had reached an agreement in principle.

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“I don’t want to get ahead of the folks who are working on it day to day,” said Richard Fields, the track’s largest shareholder, in an interview Wednesday. “I can tell you though that everybody is really optimistic. There’s a sense that everybody is really comfortable this thing won’t take much longer.”

Menino’s spokesperson Dot Joyce said today: “There is not a deal at this point – when there is a final agreement we will make it available for the public.”

Suffolk Downs, which straddles the city line between Boston and Revere, has already struck a deal with Revere. The details have been kept confidential so both agreements can be released at the same time.

The East Boston horse track is one of three competitors for the Greater Boston casino resort license; the others are Wynn Resorts and Foxwoods, which is planning a casino in Milford. Foxwoods has not yet reached a deal with the town.