If it it was written into your contract or staff handbook when you first started work, your employer may think it is entitled to impose this kind of penalty. However, whether such a penalty is proportionate, and therefore enforceable, is less certain.
Relevant factors will be whether the employer makes allowances depending on the reason for the lateness (eg arranging childcare) and the losses that the employer suffers from lateness.
If your employer is seeking to introduce a condition like this, it would count as a change of contract. See our section on contract changes for more information.
