Portland Music Holdings, the development group behind a proposed Live Nation-backed music venue in Portland, is preparing to sue the city following its decision last month to institute a buffer that blocked the project from moving forward.

Attorneys representing the group, made up of Mile Marker Investments and partner Live Nation, sent a litigation hold notice to Portland officials on May 14, requesting that staff and elected officials retain all documents related to the project in anticipation of a lawsuit being filed.

As of Thursday afternoon, the suit had not yet been filed.

The notice, sent by Portland law firm Marcus Clegg, claims that the city violated the constitutional rights of Portland Music Holdings, including “the city’s deprivation of PMH’s vested property rights without just compensation.”

Attempts to reach representatives from both Mile Marker Investments and Marcus Clegg for comment Thursday were unsuccessful.

The City Council’s April 27 decision to require a buffer between large music venues capped months of intense debate over the venue proposed at the corner of Cumberland Avenue and Myrtle Street next to Merrill Auditorium. The plan faced significant resistance from members of the city’s music scene and residents concerned about increased traffic and safety near City Hall.

The 5-4 vote to institute the 750-foot buffer came after roughly three hours of public comment, debate and amendments. It was viewed by opponents as a historic and vindicating win for those who had voiced concerns over Live Nation’s predatory business practices. Supporters, however, saw the decision as a vote against Portland’s economic future, particularly as downtown businesses struggle.

Mayor Mark Dion, who voted against the buffer last month, said Thursday he was “not surprised” to learn litigation was forthcoming but declined to comment further.

Representatives from Mile Marker Investments have previously said the buffer was simply designed to kill the venue, which otherwise complied with all city zoning regulations. Leading up to the council’s vote, the Portland Planning Board unanimously recommended against the buffer, with board Chair Joseph Zamboni stating he believed a court would find it arbitrary.

Councilors who voted against the buffer largely agreed that making retroactive zoning changes is unfair and said Portland should remain predictable for developers and businesses looking to invest in the city. But some, like Councilor Ben Grant, pushed back against that notion.

Grant, an attorney, argued that the “notion that government should run on autopilot” is contrary to what he believes about the democratic process, and that he was elected by voters to “use my judgement to do what is best for Portland.”

The decision received national attention as an example of a city saying no to Live Nation in the wake of a federal lawsuit that found the concert giant and its subsidiary Ticketmaster guilty of violating antitrust laws.

After the council vote in April, the Maine Music Alliance, which organized local opposition to the project, began fundraising almost immediately in preparation for a potential legal battle.

Scott Mohler, the group’s executive director, said Thursday that he expected the issue to end up in court, but that the grassroots effort to resist the project is “not going anywhere.”

“If they think that suing the city to bring an adjudicated monopolist into our community will endear them to the people they want to be buying their tickets, I think they’ve miscalculated,” he said. “Some people are just not used to hearing no and it shows.”

The Maine Music Alliance and its supporters, including musicians and employees of several independent music venues in Portland, held a celebration Wednesday of their effort at Geno’s Rock Club on Congress Street, attended by roughly 100 people. They vowed to continue to fight if needed.

Mohler said one byproduct of the campaign against the Portland Music Hall venue is how it has brought the city’s music scene together.

“Sometimes it’s OK for the little guy to win,” he said.