I worked for a large, international financial data company that was busted for something very similar. In essence they had a large amount of contract employees on premises (hired through a temp agency) that had worked for them for years, at desks amongst the regular employees, with the same email addresses as the regular employees, etc. They were indistinguishable from regular employees except they did not get pensions, paid leave, or any of the other perks of regular employees.
When a few were made redundant due to cutbacks, they promptly lawyered up and were able to settle for a year's worth of compensation, which is what they would have received as regular employees.
In short, if you're a contractor but have a desk in the office and an email address with the company in some cases you are more or less an 'employee', and this has been backed up by courts in the US from time to time.
http://employeeissues.com/ic_vs_employee.htm