A while back, I wrote a (not-so-quick) post on Bill 139, Ontario's new-ish legislation that extends the protections of the Employment Standards Act (ESA) to temporary workers. I don't have a lot to add right now (nor the time to do it), but a new wrinkle has come our way.
The Canada Revenue Agency (CRA) has their own thoughts on the matter of when an independent consultant becomes an employee, and thus, when they will be taxed as such, when they will have to contribute to EI and CPP, and when a company will be required to withhold taxes and contribute to EI and CPP. Like the Ministry of Labour, CRA isn't too specific about what constitutes an employee vs. what constitutes an independent contractor. Thankfully, they are more specific the Ministry. Further, they take into account the intent of both the consultant and the firm, something conspicuously absent from the information released by the Ministry of Labour.
From what information I have received, it seems that with Bill 139 there will be an inquiry only if one party complains. Which means that as long as a firm and their consultants are happy, no one will be bothered. However, it's a different beast with the CRA. They can launch an investigation without prompting, and they have a financial incentive to find people to be employees.
As well, there's no way to know what implications a CRA finding will have on a Ministry of Labour classification, or vice versa.
So far, we've never had an issue, but there's no way to know how long that will last.
You can read about the CRA's discernment process here (pdf).