Six years after the Pilgrim Nuclear Power Station in Plymouth sought to renew its license, the chairman of the Nuclear Regulatory Commission voted Thursday to deny the plant the right to continue operating for another 20 years.

His vote will probably be in the minority, and opponents of renewing Pilgrim’s license said they expect the full commission to outvote Gregory B. Jaczko, the controversial chairman who announced his resignation this week. Jakzo’s vote was widely seen as a protest of the commission’s stance on the Pilgrim plant.

In the license renewal process, the commissioners submit decisions in writing that are tabulated by a secretary and released together. But Jaczko — who has been an outspoken advocate for reforms and a lightening rod for criticism in the industry — made his vote public.

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He urged his fellow commissioners to delay their votes while litigation brought by opponents of the plant remains unresolved and the Atomic Safety and Licensing Board continues to hear appeals.

Support from the NRC while such issues remain outstanding is unprecedented, Jaczko said in a statement supporting his vote.

“While I appreciate the need to have an appropriate procedure for bringing this process to completion, the current approach that my colleagues on the Commission support is unprecedented,” he wrote.

He added: “This hardly seems to be a fair process for the petitioners” arguing that the plant is unsafe.

Jaczko announced his resignation Monday after a tumultuous three-year tenure in which he pushed for sweeping safety reforms but was criticized for an overly aggressive management style.

Neil Sheehan, a spokesman for the NRC, said the six years commissioners have spent considering the fate of Pilgrim is more than any other nuclear plant in the country.

“These proceedings were never designed to continue indefinitely,” he said.

Sheehan said that a three-judge panel of the Atomic Safety and Licensing Board recently rejected some of the opponents’ contentions about the impact of the plant on the environment.

“With Pilgrim’s license due to expire next month, there’s nothing that would prohibit a decision being issued on a renewed license at this point,” he said. “This has been by far the longest licensing renewal review. It has received a vast amount of attention from the NRC staff. To say it didn’t receive an inordinate amount of attention from the NRC staff would be at odds with the facts.”

The NRC has never rejected a license renewal application of a nuclear plant. It has previously renewed the licenses of 72 of the nation’s 104 commercial nuclear reactors, Sheehan said.

But it is not the first time Jaczko has dissented. In the past two months, he was the lone vote of the five commissioners against renewing licenses for plants in South Carolina and Georgia, arguing that more needed to be done to maintain safety in the wake of the massive earthquake and tsunami that triggered one of the world’s worst nuclear disasters at a plant in Japan.

Jaczko’s decision comes less than a month after the NRC staff recommended that commissioners vote to renew Pilgrim’s license.

The staff, in an eight-page document, said the 40-year-old plant has “taken, or will take, appropriate actions to manage the effects of aging during the period of extended operation.’’

Entergy Corp., the Louisiana company that has run the plant since 1999, has sparked a raft of protests and lawsuits since it sought to renew its operating license. Company officials did not return calls for comment.

Pilgrim opponents argue that Entergy cannot do enough to ensure safety, given what they view as the intrinsic danger of nuclear plants, especially one 35 miles from Boston and with nearly 5 million people living and working within a 50-mile radius.

They contend that commissioners should not renew Pilgrim’s license because of aging pipes beneath the plant that may leak radioactive liquids, problems with electrical cables that transmit power to and from the plant, and the lack of a sufficient cleanup plan in the event of a radiation leak.

“All the NRC commissioners except Chairman Jaczko have caved to industry and political pressure and abandoned the NRC’s own procedure that requires hearings on a license renewal application is completed before license renewal is granted,” said Mary Lampert, director of Pilgrim Watch, who has long called for the plant to be closed.

US Representative Edward J. Markey, a Malden Democrat who opposes the plant, said Jaczko’s vote against renewal shines light on the commission’s decision to take action before litigation is complete, and all appeals are heard.

“This vote is an unprecedented subversion of the rules governing relicensing of the nation’s nuclear reactors,” Markey said in a statement. “It is the latest in a long series of votes that demonstrate a reckless disregard for safety and the public on the part of [the other] commissioners.”

On Thursday, President Obama nominated Allison Macfarlane, an associate professor at George Mason University in Fairfax, Va., who served on a presidential commission that studied new strategies to manage nuclear waste, to replace Jaczko.

Pilgrim’s license is set to expire June 8.