The Carbon Free Boston Working Group that Mayor Martin J. Walsh set up to combat climate change is pushing a $5 “congestion fee” to cut the number of cars in downtown areas, but City Hall is double-clutching on the hot-button plan.

The Boston Green Ribbon Commission and Boston University report, unveiled Tuesday, suggests charging $5 “for every trip made in a private vehicle that starts or ends within” Downtown, Back Bay, the Seaport and the Longwood Medical Area, among other recommendations designed to promote walking, bikes and the T while also targeting buildings’ energy use.

“This amounts to $10 to $15 per day to drive within the congestion area, depending on the number of trips made. This fee is at the high-end of the range of congestion fees in other major cities in the world,” researchers wrote, adding that the average driver in those four areas makes an estimated 2.3 trips per day. For comparison, London has a $14.50 daily charge for drivers traveling within the city’s congestion zones.

Walsh signaled he isn’t ready to commit to the controversial concept just yet.

“Mayor Walsh appreciates the recommendations made in the report. However, the City of Boston is not implementing congestion pricing at this time,” said Walsh’s press secretary Samantha Ormsby — though Walsh is supporting the group’s general goals and the City Hall statement did not rule out implementing the proposed fee at some future time.

In 2016, Walsh called on the Commission to establish a working group that would create a detailed report with guidelines for the city to be carbon free by 2050. Chris Cook, Boston’s chief of environment, energy and open space, said the report’s guidelines are specific to the city itself.

The Boston report also suggests a $5 parking fee placed on every trip made that ends at a location other than a personal residence. Ride-hailing companies and their riders would also be affected if the city does choose to implement the report’s guidelines of a $1 per mile fee that would be imposed on ride-alone trips.

“A fee placed on private vehicles traveling in Boston will generate revenue that can fund a multitude of actions to reach carbon neutrality and make the city more resilient in the face of climate change,” researchers wrote in the 120-page report.

The congestion fees — aimed at lessening traffic and reducing fuel use and air pollution with the goal of thwarting global warming — quickly got a cool reception. If implemented, the fees could target anyone driving into Boston to go to medical appointments and visit patients; people who want shop or dine in the Back Bay or the Seaport; and anyone doing business downtown. That’s raising fears that people will choose to take their business elsewhere.

“I have a record of leading and supporting pro-environment causes,” City Councilor At-Large Michael Flaherty told the Herald. “But not at the sake of our working-class families. This is overly bureaucratic. Charging $5 for people to come in and out of the city, would have grave consequences. The cost of living in Boston has become unlivable for too many families and individuals.”

City Councilor Matt O’Malley called the report “one of the best deep dives any city could have taken,” and applauded its thoroughness. “The overarching theme of the report is that we have 12 years to get it right.”

But O’Malley, who chairs the council’s Committee on Environment, Sustainability, and Parks, said any effort to cut congestion should start with improving the MBTA.

“The focus needs to be working with the state,” he told the Herald. “We could have a lot more people riding the T.”

Meanwhile, Jon Hurst of Retailers Association of Massachusetts said, “How does it affect their delivery, local employees and customers? … We wouldn’t reject it out of hand, but it is something that would have to be piloted and studied. If it ultimately is going to be a detriment to people shopping and buying, it can be counter-productive.”

Downtown Boston Business Improvement District CEO Rosemarie Sansone struck a neutral note on the report, saying, “I will be sharing and discussing it with our members to determine how they would like to be involved in this important conversation as it moves forward.”

In June 2017, Walsh sharply rebuked President Trump for withdrawing from the Paris climate agreement, calling the rollback “irresponsible” and saying the decision would put “future generations at risk to the threat of climate change. Boston will not stand by given what’s at stake.”