Is it permissible to start overtaking without first ensuring that the driver behind you has not already begun their overtaking manoeuvre?

In the photo, in the left mirror, you can see a dark car driving right behind you, shifted towards the centre line of the roadway. You are not allowed to start overtaking without first making sure that the driver behind you has not started overtaking. The answer is: no.

The Road Traffic Act states that before overtaking, the driver of a vehicle is obliged to make sure, in particular, that he has adequate visibility and enough space to overtake without obstructing anyone, that the driver behind him has not started overtaking, and that the driver in front of him in the same lane has not signalled an intention to overtake, change direction or change lanes. These are mandatory conditions, checked before every manoeuvre, not recommendations: starting to overtake without looking in the mirror and over your shoulder is a breach of the rule, even if nothing happens. The car in the mirror is close and has moved to the left - this is the typical picture of a driver who is just pulling out from behind you; if you moved into the left lane now, you would cut him off. You wait until he passes you or moves back to the middle of the lane, and only then do you signal and overtake.

Legal basis: Art. 24 para. 1 point 2 and point 1 and 3 of the Road Traffic Act (Prawo o ruchu drogowym)