Your employer may add a probationary period as a term of your contract starting from day one at work and lasting a number of weeks or months.
Your contract should state how long the probationary period will last and whether it can be extended.
Some employers will offer employees less favourable terms and conditions and a lower salary during the probationary period. They can do this as long as it is written in the contract. However, they cannot take away your basic statutory rights. Also, you should be treated as an employee from day one.
Your continuity of employment should start on your first day of work, not when the probationary period has ended. This becomes important if your employer uses continuity of employment (i.e. length of service) for certain entitlements such as annual leave.