Despite blunt warnings from Mayor Carlos Gimenez that their decision would force layoffs of hundreds of police and corrections officers, Miami-Dade commissioners Thursday narrowly refused to force two employee unions to contribute an additional 5 percent of their pay toward health insurance.

Facing a chamber packed with police officers, the commissioners voted 7-6 against settling a labor impasse between the county and the Dade Police Benevolent Association by forcing the additional concession on 5,400 police and corrections officers.

The panel also refused to impose the controversial 5-percent giveback on its professionals and supervisors, who are represented by the Government Supervisors Association of Florida OPEIU Local 100. That move spells an almost-certain wave of layoffs for them as well.

“It’s an absolute lack of political courage by a board that wants to have its cake and eat it too,’’ the mayor said at the conclusion of a nine-hour meeting. “I’m disgusted at what happened today.’’

The mayor estimated that at least 154 police officers and 145 corrections officers would have to be laid off as the county scrambles to close a budget gap it had expected to fill through additional employee concessions. The county will also likely have to close a corrections facility.

The mayor said he and his staff would meet Thursday night to begin hashing out how to fill the $35-million budget gap that he had envisioned eliminating with the additional health-care contribution. He said other options have already been exhausted.

The mayor said commissioners knew that painful employee concessions were needed when they voted in September to approve his budget plan, which reduced the property-tax rate. “When it came down to making the tough decision, they buckled,’’ he said.

Commissioners offered various explanations for their reluctance to defy the unions. Commissioner Jose “Pepe’’ Diaz said he couldn’t force the concession on police after attending funerals of slain officers and knowing how tough their jobs are.

At one point, Diaz asked the officers in the audience if they would rather swallow the concession or reject it and see fellow officers laid off. “Do it!’’ several officers shouted, with others echoing support of the layoffs.

The tortured vote by commissioners, who second-guessed their decision before sticking by it, came after nearly seven hours of pleas by the labor unions that are traditionally among the politicians’ most reliable fundraisers and voting blocks.

Half of the commissioners — those in odd-numbered districts — face re-election contests in August.

The seven commissioners who voted against the increased healthcare contribution were Bruno Barreiro, Diaz, Barbara Jordan, Jean Monestime, Javier Souto, Xavier Suarez and Chairman Joe Martinez. Voting for it were Lynda Bell, Esteban Bovo, Sally Heyman, Dennis Moss, Rebeca Sosa and Audrey Edmonson.

Edmonson said she supported the measure because she did not want to see police officers laid off. “I cannot vote for adding to the unemployment’’ rate, she said.

The PBA, which bargains for police and corrections officers, and the GSAF, representing nearly 4,600 professional employees and supervisors, recently approved new labor contracts that include significant cost savings to the county, but the parties hit an impasse on doubling the health insurance contribution to 10 percent of base pay.