What to do about an intruder?

[Optimist1]

I have two cats - neutered male and female from same litter - now coming up for 15 years old. They've always been free to come and go in my house, using cat flap in the back door.

I noticed a couple of weeks ago that the female was peeing in odd places around the house. No signs of her being off-colour but I think it might have been a stress thing because one evening I discovered another cat making itself at home in my kitchen. I chased it out, and it scooted off, but has been bold enough to come in again once or twice (to my knowledge) since then. It appears to come in to finish up the leftovers from my cats' food, but is obviously well fed and has a collar on.

Any bright ideas for me, please? (I'm averse to locking the cat flap as their whole life experience has included being free to come and go as they please.)

Microchip controlled cat flap, Optimist1. That will read their chips and only allow them in - not the intruder. A bit more expensive than other flaps but well worth it for security. Many posters on here have them.

PS - and if they haven't got chips, that's worth it also if they go outside in particular.

Thanks, Cozie. They are chipped already. I'll google the catflap you mention.

As I was writing OP it also occurred to me that I would be wise to take away their food bowls soon after they've finished eating - they'll just have to abandon their "grazing" habit for a little while.

I think Sureflap is the one most posters use - have a look for that. Should cost somewhere around £70.

I had intruders and we got the sure flap installed. The cats have chilled out a lot. Totally recommend it and don't why I didn't do it earlier.