New York will require all private-sector workers in the city to be vaccinated against Covid-19, in what would be the strictest vaccine mandate to be imposed anywhere in the US.

Mayor Bill de Blasio said on Monday the new rule would go into force on December 27 and would apply to all private companies regardless of size. His office will release a set of protocols applying to about 184,000 businesses on December 15 providing further details of the mandate, such as enforcement and penalties for non-compliance.

De Blasio said there “will not be a testing option as an alternative” to being vaccinated, as was being proposed by the Biden administration as part of a separate nationwide plan.

The mayor said the mandate was being imposed to slow the spread of the Omicron coronavirus variant that had threatened to exacerbate infection rates in the city, which had almost doubled in the past month.

He told the news channel MSNBC: “We in New York City have decided to use a pre-emptive strike to really do something bold to stop the further growth of Covid and the dangers it’s causing to all of us.”

De Blasio added: “Omicron is here, and it looks like it’s very transmissible. The timing is horrible with the winter months.”

Business groups could appeal against the New York ruling in the courts. De Blasio and Georgia Pestana, the city’s top lawyer, said the health commissioner had a legal obligation to protect the public health of New York City and they were confident the mandate would survive any challenges in court.

Devjani Mishra, an employment lawyer at Littler Mendelsohn, called it a “first of its kind requirement in terms of covering all private employers”.

She added: “The enforceability of this mandate rests on both the fact that the pandemic is a general public health emergency and the fact that the mandate will apply ‘across the board, not picking and choosing among industries.”