UK Car Repair: Cant switch the car off, audi a4, likely culprit

Question
Hi, hope you can help. I have a Audi A4 , year 2000. I have had car serviced and so on, however I have a problem with switching it off. I have changed the ignition switch as it was possible it was causing it however that did not fix the problem. Everything else was checked around to see, the hands free, the stereo and so on, however that is not causing it either. No one is able to tell me what it could be and the problem is serious. The only way to shut the engine off is by tapping on the brake, otherwise you can drive the car without the key in at all. Did anyone experience anything like this before and could they tell me where I need to approximately start looking for a problem?
Thank you all in advance!

Answer
Sorry for the delay but this is an unusual problem which I had to research. In spite of that I'm not absolutely sure of where the problem is. Other than a faulty ignition switch it would appear the likely culprit is the "Starter load relay". this controls the feeds to all other sources-including the brake lights- under starting. The only way to confirm is to trace the feeds physically!