Four men died when the scaffold collapsed on Christmas Eve in 2009. (CBC)

An Ontario construction company could be fined as much as $1 million after pleading guilty to criminal negligence causing death.

Four construction workers fell to their deaths from a scaffold on Christmas Eve 2009. It was the worst construction accident in Toronto in decades.

Metron Construction admitted negligence because a supervisor working at the Kipling Avenue apartment building allowed six people onto the scaffold. Ontario safety regulations say there should only have been two.

The scaffold collapsed late in the afternoon on December 24, killing four men.

Only one of the six men on the scaffold was attached to a safety line as required by law. Another man survived the fall but suffered serious injuries.

Labour lawyer Jeremy Warning say the case is important because it is the first time in Ontario that a company has pleaded guilty to criminal negligence for a workplace accident.

In their sentencing submission prosecutors are seeking a $1 million fine.

"We'll have to see what sentence is ultimately imposed to determine how precedent setting it is," said Warning.

The owner of the company, Joel Schwartz, has had charges of criminal negligence causing death against him dropped by prosecutors who say they do not believe there is a reasonable chance of a conviction.

Court documents show that three of the four men who died had marijuana in their systems before going up to repair the balconies. One of the three was a supervisor.

Allowing drugs on the job was another reason why Metron Construction was found criminally negligent.

The case resumes at Old City Hall later this week.