The DUP has been accused of a "political ambush" and misusing power after it used Assembly rules to block rules designed to protect minorities in local government.
It means Northern Ireland's 11 new super councils will go 'live' in April without built-in protections for nationalist and unionist minorities in their local areas.
The DUP tabled a petition of concern which requires a majority among both unionist and nationalist MLAs, allowing an automatic veto because it is the only party with enough MLAs to trigger the mechanism without support from others.
The party used the petition despite criticising Sinn Fein for using the same tactics to block its planned 'conscience clause' arising from the "gay cake" controversy. But it came under sharp attack in the Assembly for launching its veto at the 11th hour, after the regulations spent weeks under scrutiny before Assembly committees and in public consultation.
The SDLP's Alban Maginness said it would prove highly damaging to the way in which the new councils will operate.