The Seattle City Council defied the mayor to let Uber drivers unionize, and now he won't sign the bill

Seattle made history as the first city to pass any legislation on Uber drivers unionizing, but Mayor Ed Murray isn't having it.

The Seattle mayor sent the city council a letter urging it to seriously amend the legislation before it voted. The council ignored it, and voted Monday to unanimously approve the bill.

The approved legislation will allow Seattle drivers join a nonprofit that qualifies as a "Driver Representative Organization" (like a union). Uber, Lyft, and other companies will have to send the nonprofit list of eligible drivers from Seattle, who it could then contact. If the nonprofit could show that it had the support of a majority of drivers, then the city would have to designate it as a bargaining representative.

The non-profit will then be able to argue for things like minimum wage and paid time off, although those aren't guarantees.

"Today, we set an example for how cities across the country can work to ensure safe, reliable transportation and basic worker protections in our current legal landscape," City Council Member Mike O'Brien, who first proposed the ordinance, said in a press release.

However, the city's mayor is none to happy about it.

Before the vote, he sent the city council a layer outlining his protests, which included "flaws" of who and how the city will determine as a "qualifying driver" and the governmental expense.

"Most notably, City costs of administering the collective bargaining process remain unknown and the Council has placed the burden on significant rule-making on City staff," he wrote to the council in a letter obtained by Business Insider.

After the vote, which passed unanimously, the Mayor has decided not to sign the bill. It will become a law anyway - the mayor could have vetoed it, but the council passed the ruling unanimously, so they would have been able to override his veto.

"Since my concerns were not adequately addressed in this legislation, I will not sign this bill," he said. "Under the City Charter, the ordinance will become law without my signature. As this ordinance takes effect, my administration will begin its work to determine what it will take to implement the law. I believe it will be necessary to seek additional clarifying legislation from the Council. I look forward to working with council members in 2016 on their ordinance."