The Barclays Center has taken the first preliminary step toward a potential opt-out of its 25-year deal with the Islanders, according to sources.

Brooklyn Sports & Entertainment, which runs the Barclays Center and the renovated Nassau Coliseum, recently notified the Islanders in writing that the window to renegotiate the terms of the license agreement is open, as per the terms of the contract.

The two sides have until Jan. 1, 2018, to renegotiate the terms, according to the summary of the license agreement previously obtained by Newsday. If no new deal is reached, the two sides can stay with the current deal or choose to opt out. Each side would have until Jan. 30, 2018, to deliver an opt-out notice in writing.

If the Islanders decide to opt out, the team can choose to leave at the end of next season or the end of the 2018-19 season. If Barclays triggers the opt-out, the Islanders would have to leave after the 2018-19 season.

The sources said the Islanders and Barclays Center had until 30 days after the Islanders’ season to trigger the start of the renegotiation period; Tuesday marked the 30th day since the Islanders’ final game of the season.

Both the Islanders and Barclays Center declined to comment.

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Although the letter was described by the sources as a procedural step, it’s an important one. According to a lease summary document, the two sides must engage in “good-faith discussions” to renegotiate the financial terms of their agreement in order for either side to opt out of the deal next January.

NHL Commissioner Gary Bettman told reporters last month that the Islanders expect to submit a bid to the state later this year to build a new hockey arena at Belmont Park in Elmont.

Empire State Development, the state’s primary business development agency, is “working to finalize” a new request for proposals to develop the state-run property in Elmont, according to agency spokeswoman Amy Varghese.

Last year ESD scrapped proposals, including a 25,000-seat soccer stadium for the New York Cosmos, after a long-delayed, four-year process.

The operators of Barclays Center, meanwhile, are expected to pitch a plan for the Islanders to return to their former home at the Nassau Coliseum, which reopened last month after a $165-million renovation, according to Kevin Law, president and chief executive of Long Island Association, the region’s largest business group.

A spokesman for Nassau County Executive Edward Mangano, who has lobbied the Islanders to return to the renovated Coliseum, did not respond to a request for comment.