The Salem and Beverly commuter rail stations, two of the busiest in the MBTA system, are losing more than 300 parking spaces on Monday.

The T announced yesterday that it will reduce parking at the Salem station by about 225 spaces to begin archaeological fieldwork that is part of required environmental permitting for the new MBTA garage. The spaces won’t be available for six to eight weeks, the T said.

It put signs up at the station this week announcing the plans.

Also on Monday, the MBTA will close its 100-space lot on Court Street near the Beverly train station to begin construction of a 500-car parking garage. That closing was announced at an Oct. 19 public meeting.

Although it wasn’t planned this way, the two busy parking lots are being affected on the same day to keep the long-awaited Salem and Beverly station construction projects on schedule, the T said.

In Salem, the MBTA plans to build a 715-space garage as part of a $37 million station upgrade. Work is scheduled to begin in the spring.

“The work is being performed simultaneously in order to satisfy the schedule commitments made to the public for each project,” MBTA spokesman Joe Pesaturo wrote yesterday in an email.

“The MBTA apologizes for this temporary inconvenience while we work to provide improved facilities in both Beverly and Salem.”

Although the Beverly parking lot closing has been known for a few weeks, the Salem decision was announced to the general public only yesterday due to unforeseen scheduling issues, officials said.

The MBTA received approval just days ago from the Massachusetts Historical Commission to excavate and document the historic Salem roundhouse, which stood on the site of the Salem station a half-century ago and the remains of which are buried under the current parking lot.

“This archaeology work is a necessary and important component of the MBTA’s environmental review and environmental permitting requirements, all of which must be completed before we can begin construction on the new station,” the agency said in a statement.

Salem Mayor Kim Driscoll said she was told they must do the work now to keep the project on schedule.

“I think, ideally, they would like to be able to give more notice, but they just got the sign-off from (the Massachusetts Historical Commission), and they’re trying to beat the weather,” she said.

Driscoll said she thinks the parking spaces will be lost for about a month, not as long as two months.

The Salem station will still have parking for the remainder of the year, but much less than it does now. Currently, there are about 460 spaces — 340 on the MBTA’s section of the lot and 120 in a smaller city section.

For the next few weeks, there will be a total of only 235 spaces.

The work in Salem will not affect disabled commuters, the MBTA said.

“None of the accessible parking spaces or pedestrian walkways will be affected by the work at the Salem station parking lot,” Pesaturo stated.

Alternative parking is available in the 900-space Museum Place Mall garage, or in other city lots.

The Salem Parking Department, located on the ground floor of the Museum Place Mall garage, sells discount parking stamp books.

In Beverly, officials are advising commuters to park in the municipal lots along Cabot Street. Mayor Bill Scanlon said signs will be in place Monday to point out the location of those lots.

Parking is also available at the North Beverly station, Montserrat station, Swampscott station and the Lynn MBTA garage, the MBTA said.

Staff writer Paul Leighton contributed to this article.

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