The seven new stations for the embattled MBTA Green Line extension into Somerville and Medford are now scheduled to open in 2021, pushing back the long-delayed project yet again, state transportation officials said Wednesday.

When the federal government pledged $1 billion to the project in late 2014, the Union Square and Washington Street stations were scheduled to open in 2017, with the other stations scheduled to open in 2020. But rising cost estimates prompted the state to halt the project and complete a scaled-back redesign.

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