The result tightened Mr. Corbyn’s grip on the party and isolated many of its members of Parliament from a growing membership that is younger and more left-leaning, drawn by Mr. Corbyn’s policies to reduce inequality, make Britain non-nuclear and renationalize key areas of the economy, like the railways and energy.

The party has almost tripled its membership to more than 500,000, making it the largest political party in Western Europe, Mr. Corbyn said. But opinion polls regularly indicate that if an election were held tomorrow, Labour under Mr. Corbyn would suffer a historic defeat in the country as a whole.

In a victory speech, Mr. Corbyn called for unity, said that more held “the Labour family” together than divided it and vowed that the party would win the next election under his leadership.

Mr. Corbyn also promised forgiveness to the rebels, noting that many heated things are said in a campaign that are later regretted. “Let’s wipe that slate clean from today and get on with the work we have to do as a party together,” he said.

There are concerns among the rebels in Parliament that Mr. Corbyn and his team will move against them. Electoral districts are to undergo boundary changes as the House of Commons shrinks from 650 members to 600, and Mr. Corbyn’s opponents fear the leadership team will use those changes to replace them with other candidates. Some of Mr. Corbyn’s allies have been pressing for “mandatory reselection” of all candidates in every district before another British election.