SOUTH PORTLAND — The Boston law firm Foley Hoag will defend South Portland against a lawsuit filed last month by Portland Pipe Line Corp.

The City Council unanimously approved the hiring Monday night.

In the nine-count lawsuit, Portland Pipe Line and American Waterways Operators allege the city’s Clear Skies Ordinance, enacted last August, unconstitutionally limits Portland Pipe Line’s ability to operate to its full capacity. The ordinance “reduces the current market value of (Portland Pipe Line’s) pipelines and hinders its ability to engage in interstate and international commerce,” according to the lawsuit.

The Clear Skies Ordinance was drafted to prohibit the bulk transfer of unrefined tar sands onto marine vessels and other Portland Pipe Line infrastructure within city limits. Limiting such transfers is outside of the city’s jurisdiction, the lawsuit alleges.

The decision to hire an outside firm was criticized by some members of the public.

“Why aren’t we buying local? Why are we going all the way to Boston to hire a firm?” Warner Ogden asked councilors at Monday’s meeting.

Sally Daggett, an attorney with Portland-based Jensen Baird Gardner & Henry and the city’s corporation counsel, said the decision to hire Foley Hoag was “attractive” to the city for a number of reasons, including, “they know the turf about what it means to represent a public entity.”

“Given the nature and the extent of the claims,” Daggett said, “the city thought it made sense to get a fresh perspective” from a firm that has a “national energy and environmental practice.”

Moreover, the city searched for a firm that had “no actual or positional conflicts with the oil industry,” Daggett said, including environmental firms that opted not to take a direct position against the oil industry.

The 250-attorney firm was selected as top environmental firm in Massachusetts by Chambers USA in 2013.

Where, specifically, the city will get money to pay for the firm throughout the litigation, which could last several years, was not discussed.

But a fund was set up to collect donations to fund litigation costs, Mayor Linda Cohen said.

“No amount is too small,” she said.

City Manager Jim Gailey also told the public that the city will be exploring “alternative ways of funding or (collecting) donations toward this cause.”

Ogden criticized the decision, stating it is “fiscally irresponsible to approve hiring a law firm without an estimate of price.”

Other residents viewed the costly process as the lesser of two evils and said there is value in the expenditure, at least for the time being, as a sacrifice to keep South Portland environmentally safe.

Andy Jones told the council he was proud of the steps South Portland is taking toward environmental sustainability.

“As far as environmentalism, as far as clean air, South Portland isn’t just talking the walk, it’s walking the walk,” Jones said.

“We are being tackled by a very large organization which is international, which I read recently has a higher credit rating than the United States government,” another resident, Bob White, said. “That is who’s coming after us. We need to fight that. They are trying to overturn a democratically enacted act of self-governance.”

Catherine Chapman said she is retired and on a fixed income but “can’t see anything that would be more important than to be fighting this for our health and welfare in the city.”

“I say bravo, and keep going, and we’re right behind you,” Chapman told the council. “We’re moving forward, and we’re not going to let anyone stand in our way.”