The opening of the MBTA's Green Line Extension will be delayed by several months

The years-long wait for the Green Line Extension just got a bit longer.

MBTA officials said Monday that full service on the 4.7-mile project extending the Green Line to Somerville and Medford won’t begin until next spring, five months after the extension’s previously scheduled opening, primarily due to delays caused by the COVID-19 pandemic.

Under the new timeline, the Green Line Extension’s one-stop branch to Union Square in Somerville — previously set to open in October — won’t open until sometime in December.

Meanwhile, the longer, five-stop branch through Somerville to Medford and Tufts University won’t open until May 2022, five months after its previous end-of-year completion date.

“There always has been schedule pressure,” John Dalton, the Green Line Extension project manager, said during a meeting Monday, adding that it was only “exacerbated” by COVID-19.

MBTA officials had maintained over the course of the last year that the entire $2.3 billion project remained on target for completion, after supply chain issues began to emerge last spring. Gov. Charlie Baker even teased agency officials in October that he would put the end-of-2021 completion date for the long-stalled project — initially planned in the 1990s as part of the Big Dig — “up on his wall.”

However, Dalton said Monday that the supply chain impacts they heard about last year “are now kind of revealing themselves, as far as what materials arriving on site and being installed and paid for.” State officials said last week that GLX crews are scheduling at night work during deal with the sporadic arrival on backlogged items like noise wall panels.

The revised timeline was approved by the MBTA’s Fiscal Management and Control Board as part of a settlement agreement with GLX Constructors, the group of companies contracted to build project, to reduce the MBTA’s risk of facing lawsuits related to COVID-19 and “pin down schedule certainty.”

Under the agreement, the MBTA will pay up to $80 million from the project’s built-in contingency funding budget to insulate itself from potential legal liabilities, pay for pandemic-related costs, and set contractor staffing requirements for the remainder of the project. T officials said last month that the Green Line Extension, which had been under budget, had over $300 million for unexpected contingency cost increases.

“We’ll be closing out all COVID impacts — both cost and schedule — through this settlement,” Dalton said, noting that the project is in a “very solid financial position.”

Somerville Mayor Joe Curtatone, who spoke during the meeting Monday, acknowledged the disappointment of the delayed opening

“I’d be the first to say I’d like to have it all today,” Curtatone said of the project, which is set to increase the number of Somerville residents who live within a half mile of a MBTA subway stop from 20 percent to 80 percent.

But he stressed that the project — being built through densely populated cities amid a pandemic and adjacent to other construction projects like Cambridge Crossing and the new Somerville High School — is “not just laying down tracks.” Curtatone also applauded the MBTA’s “sound management” of the project and the settlement’s “tremendous” long-term impact.

“This is a complex project, but a great one,” Curtatone said.