The parent company of Central Maine Power is suing over the constitutionality of a referendum aimed at overturning decisions by three state agencies that approved a $1 billion utility transmission project.

The lawsuit, filed Tuesday, contends the Maine Constitution links referendums to legislative acts, not agency actions. It also contends the referendum violates the state constitution’s separation of powers.

The New England Clean Energy Connect would serve as a conduit for up to 1,200 megawatts of Canadian hydropower to reach the New England grid.

It has been approved by the Maine Department of Environmental Protection, Land Use Planning Commission and Public Utilities Commission. But critics obtained enough signatures to put the project to a statewide vote in November.