SOUTH PORTLAND — The city plans to hire a Boston law firm to defend its Clear Skies ordinance against oil and shipping businesses that have filed a lawsuit challenging the ban as unconstitutional, according to a news release Wednesday evening from Mayor Linda Cohen.

Portland Pipe Line Corp. filed a federal lawsuit against South Portland on Feb. 6, seeking to overturn the city’s ban on loading crude oil into tankers in Portland Harbor.

It’s unclear how much the defense will cost the city, but its general liability insurance carrier, the Maine Municipal Association, has told city officials that the policy doesn’t cover the lawsuit, the release said.

The City Council will vote Monday on a proposed legal contract with Foley Hoag, an international firm that specializes in environmental law and litigation.

“Given the nature and extent of the claims made against the city and its CEO, the city manager and the corporation counsel are recommending to the city council that the city retain the expertise of special legal counsel with an extensive energy and environmental litigation practice,” Cohen’s release said.

The city is taking steps to confirm the municipal association’s assertion that its policy doesn’t cover the lawsuit. If that turns out to be true, the city would pay related legal fees, said City Manager Jim Gailey.

Gailey and Cohen declined additional comment.

The Clear Skies ordinance aims to prevent the flow of the South Portland-to-Montreal oil pipeline from being reversed to bring Canadian oil – and potentially tar sands oil – into Maine.

In the lawsuit, filed in U.S. District Court, the company alleges that the ordinance interferes with interstate trade, discriminates against Canadian interests, devalues the pipeline and infringes on areas best left to the federal government. The company is joined in challenging the ordinance by a trade association representing tug, towboat and barge operators.

The ordinance was adopted in July, eight months after South Portland voters rejected a proposal that would have allowed only the unloading of oil in South Portland. Reversing the flow of the pipeline would enable tankers and barges to load oil from the pipeline and take it to other ports and refineries.

Opponents said that could increase the risk of spills and would require the construction of smokestacks next to Bug Light Park to burn off gases associated with loading diluted bitumen – otherwise known as tar sands oil – from the pipeline onto tankers.

Portland Pipe Line had insisted throughout the process that it had no contracts and no plans to reverse the flow of the pipeline. But the lawsuit’s comes against a backdrop of changes in energy markets that are threatening the viability of the South Portland-to-Montreal pipeline. Those changes have led to steep declines in the volume of crude oil pumped through the pipeline.