I too had this problem, even when we had the magnetic collar cat flap, intruders kept coming in and terrorising our cats, and oh god - the smell. I was highly extremly sceptical of the expensive cat flaps which electronically read your own cats' microchips, thereby only letting in your own cat - was unconvinced for years. My eventually vet persuaded me that very many of his cat owners swore by them and so I reluctantly forked out the expensive ?80 or so - as I was desperate. This has been one of the BEST ?80 I have ever spent. Despite looking flimsy, as if any big bruiser cat could barge in - the cat flap does indeed work. Only my two cats can get in, they just had to learn there is a slight delay before the reader recognises their chip, and the flap opens for them and only them. I watched the intruder try a few times - butting the flap - but as it never worked for him, he VERY soon gave up and we have never had the intruder problem since. I now realise the reason these cat flaps are expensive is because they really do work. We have the SUREFLAP one. There is a Staywell all also. Read all the reviews on Amazon etc. Fairly accurate reviews. This has been my longest post ever. Hope it helps.