After months of delays, the coronavirus vaccine mandate for Philadelphia’s unionized municipal workers took effect Friday, according to Mayor Jim Kenney’s administration.

But the union representing the city’s 2,600 firefighters and paramedics has vowed not to cooperate and plans to sue the city over the mandate on Monday, said Mike Bresnan, president of the International Association of Fire Fighters Local 22.

Kenney first announced the policy in November, but the mandate has been delayed several times as the city sought to hash out deals over its implementation with the four major municipal unions.

The administration reached agreements or secured arbitration decisions laying out how the mandate will be implemented with the unions representing Philadelphia police officers and the city’s blue- and white-collar workers — but not Local 22, which vehemently opposes the mandate and only recently began to take part in an arbitration process that could resolve the dispute.

“The city is moving forward with implementing the vaccination mandate as announced last week for all represented employees including members of Local 22,” Kenney spokesperson Joy Huertas said. “The city has notified Local 22 that it remains ready to negotiate the impact of this policy and promptly proceed to interest arbitration to resolve any disputes, as it has been trying to do since November.”

The union, however, is encouraging its members not to comply with Friday’s deadline to submit proof of vaccination or apply for a medical or religious exemption, Bresnan said.

Bresnan said the union is pursuing an unfair labor practice complaint over an email sent to city employees this week notifying them of the Friday deadline, arguing that Local 22′s contract guarantees the union the right to an arbitration panel decision before its members are subject to a COVID-19 vaccine mandate.

“That message they put out there the other day does not pertain to Local 22 members,” Bresnan said. “If that’s their stance, we’ll sue them on Monday.”

The firefighters union had previously objected to participating in arbitration over the mandate, but on Thursday it appointed a representative to an arbitration panel that will resolve the dispute, the city said.

The Fire Department is the least-vaccinated city department, with only 61% to 70% of workers inoculated, according to the mayor’s office estimates.

The agreements reached between the city and its two bargaining units represented by the American Federation of State, County and Municipal Employees included provisions that delay the start of the mandate for members of those unions if any other union secured a later deadline to comply.

Those clauses have created uncertainty over Kenney’s policy, raising the possibility of a further delay in the mandate for most city workers if the firefighters arbitration panel sets a later date.

But a spokesperson for AFSCME District Council 33, which represents the city’s 9,500 blue-collar workers such as trash collectors, said Friday that the union plans to follow the timetable laid out in the arbitration decision for the 6,500-member Fraternal Order of Police Lodge 5.

Under that schedule, city workers must submit proof of vaccination or an exemption application by the end of Friday. If they fail to do so, workers will be placed on unpaid leave for two weeks, and, if they have not begun getting vaccinated by the end of that period, they will be fired.

A separate vaccine mandate for nonunion city workers took effect Dec. 1, and 13 employees were fired, the city said.