San Francisco transportation officials have delayed the opening of the Central Subway: It will now start operating in mid-2021 instead of the end of this year, The Chronicle has learned.

The San Francisco Municipal Transportation Agency won’t substantially complete the project — moving construction from the streets down into underground stations — until next summer. Then they’ll need a year to test the system and integrate everything, said SFMTA spokeswoman Erica Kato.

“We’re targeting revenue service for mid-2021,” she said.

The $1.6 billion project, meant to bring sports fans to Warriors games at the Chase Center and tourists into one of the city’s main cultural districts, has been dogged from the time it broke ground in 2010. Critics decried the 1.7-mile extension of Muni’s T-Third line as a monumentally expensive boondoggle. The SFMTA went to battle with its contractor, Tutor Perini Corp., over claims and change orders to the project.

And delays have piled up. When the project began, officials said that passengers would be able to board trains by the end of 2018, a date they later revised to 2019. Crews used tunnel-boring machines to dig beneath Fourth Street, through SoMa, Union Square and up Stockton Street into North Beach. Merchants along that corridor complained that the perpetual street work was putting them out of business, and the city eventually had to shore them up with money and other forms of assistance.

Most recently, the Chinatown Central Subway Station was at the center of a controversy over the late Chinatown power broker Rose Pak, considered by many to be the main crusader for the project. Board directors for the San Francisco Municipal Transportation Agency voted last month to name the station after Pak, despite objections from her political enemies and people who called her a bully. Some of them have vowed to challenge the naming on the November ballot.

SFMTA had long insisted that the Central Subway would open in December, even after a project manager in April moved the date from 2019 to February 2020. In July, the agency announced a new project manager, Nadeem Tahir, and signaled that the opening could be pushed back. Tahir and acting transportation chief Tom Maguire said they would assess the project over six months to pinpoint a new completion date.

“It’s disappointing — I, like everybody else, wanted the subway to open in 2018,” said Gwyneth Borden, vice chair of the Board of Directors. She noted that delays aren’t unusual for a massive, complicated infrastructure project that requires crews to dig underground.

“Nobody is surprised,” said Supervisor Aaron Peskin, whose district includes Chinatown and North Beach. “It’s a huge bummer, but at least they’re finally telling the truth when they should have told the truth a long time ago.”

P.J. Johnston, spokesman for the Chase Center, said: “These things are beyond our control. We’re certainly eager to see the Central Subway in operation, but we’ve known for some time that it wouldn’t be open in time for the beginning of the Chase Center era. That’s why we’ve been working hard to implement a transit plan that ensures all modes of transit are being utilized, including trains coming from the Market Street subway, buses from the Mission and Civic Center (BART), ferry service, (Uber and Lyft), bicycles and walking.”

Jeff Cretan, Mayor London Breed’s spokesman, said the mayor is frustrated and disappointed that the project is again delayed.

“We know there are hardworking people at SFMTA, but San Francisco has to get better at large-scale project delivery,” Cretan said. “This includes working with the city attorney to determine where in this project’s contracting process mistakes were made to avoid seeing those same mistakes in the future. Our residents deserve better, and we will do better.”

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan