Charleston County Council narrowly approved extending I-526 across James and Johns islands Thursday, ending one of the most intense local political debates in recent memory, and opening a new chapter of court and regulatory challenges to the controversial highway.

Pitched by its proponents in terms of now or never, council voted 5-4 to build Alternative G — a 45-mph at-grade “parkway” that council had unanimously rejected in 2011.

By doing so, council abandoned any opportunity to seek a transportation solution that would not be so polarizing as extending the interstate loop.

The roadway, including two new bridges over the Stono River, will be built from its terminus west of the Ashley to the James Island connector, which links to downtown Charleston. Cost of the project currently is estimated at $558 million, to be funded by the State Infrastructure Bank. But opponents contend that it will ultimately cost far more — likely at county expense.

There will soon be a membership shift on the SIB that could have made it more amenable to the package of less expensive road solutions earlier sought by County Council, without success.

Meanwhile, there is growing sentiment in the Legislature that the Lowcountry is getting too much of the SIB pie already. The Arthur Ravenel Jr. Bridge was one of the first SIB projects.

Unlike the bridge, however, the I-526 extension is not considered a statewide priority. Indeed, it ranks only 15th on a local list of priority projects.

But council’s decision puts the project at the head of the line — assuming it can survive pending challenges over permits required to build across the river and marsh.

Meanwhile, other major local projects of statewide importance have yet to be tackled. Vital projects include widening portions of I-26, and road improvements associated with port expansion and the Boeing plant in North Charleston .

Although environmental issues will be at the forefront of future efforts to block the highway, most of the critics who have spoken against the project over the years have cited the certainty that the extension will encourage new development.

Others have opposed the notion of appending a 45-mph road with numerous at-grade intersections to the existing high-speed interstate loop, saying that it defeats the purpose of having an expressway.

Councilman Vic Rawl, who voted for the project, called it “the best we can get ... the best of the worst.”

Councilman Joe Qualey, who voted against it, said it would “create sprawl and development,” not traffic relief.

Alternative G will have six interchanges on James and Johns islands, instead of two as initially planned. The latest plan would have an overpass only at the intersection of 526 and Folly Road.

The plan was broadly opposed in five well-attended public hearings in 2010.

County Council acknowledged that opposition by unanimously rejecting Alternative G and voting 5-4 for a “no-build” option in 2011.

But a campaign led by Charleston Mayor Joe Riley convinced council to take a U-turn on the project. That effort was backed by the mayors of North Charleston and Mount Pleasant, along with the Chamber of Commerce and legislative advocates. It was assisted by a quickly prepared survey from the state Department of Transportation showing public support for extending the highway.

Council’s approval of the new I-526 “parkway” will almost certainly embolden supporters of linking a limited access highway to the resort islands of Kiawah and Seabrook.

Those opposing the expressway include Johns Island residents who see the I-526 extension as accelerating changes in a rural community that already is seeing heightened suburban and resort growth.

As for James Island, the construction of a short section of roadway, with multiple at-grade intersections, paralleling James Island Creek, is certain to be followed by new development — and lots of traffic.

Opponents who have attempted to make their voices heard over those of the politicians — and at one point thought they had — can’t help but be downcast over County Council’s change of mind.

But their continued vigilance will be necessary to forestall further development pressures to the extent they can make their voices heard — as daunting as that might seem after council’s vote last night.