Boston police detectives will get a hefty retroactive pay raise, after action on Wednesday by the Boston City Council.

With members of the Boston Police Detective's Benevolent Society looking on, the councilors voted unanimously to approve an arbitrator's award of a 29 percent raise, which covers a six-year period.

District Councilor Mark Ciommo said that while the increase is in line with other recent public safety contracts, it highlights the huge gulf between civilian and public safety workers when it comes to pay increases.

"The issues with these awards through arbitration, we're seeing 25, 26, 27 [percent raises]," he said. "While our civilian employees are getting 12.6 to 14 percent over a six-year period. It's unsustainable to do that."

Councilors also expressed concern that the pay hikes are crowding out other city spending priorities. After the vote, councilors signed a letter saying their decision "should not obscure our concern that this level of pay increase is not sustainable for the City's finances and our expectation for different outcomes in the next round of contract negotiations."

Mayor Marty Walsh agrees and said Wednesday he hopes this is the last time the city has to go to arbitration.

The council approved a similar arbitration award in 2013, voting to give the city's police patrolmen 25 percent raises over a six-year period.

Councilors could have rejected the award Wednesday and sent the sides back to negotiations. The Boston Municipal Research Bureau had urged the council to reject the award to stop the escalating arbitration payouts.

With reporting by WBUR's Delores Handy