New very territorial cat

[TammySwansonTwo]

A few nights ago both my cats ran in from outside in terror - couldn't see what had caused it.

Yesterday a white and grey cat appeared in the garden, never seen it before. It's getting increasingly confident and keeps returning, getting closer to the cat flap. We can't get a microchip flap as it doesn't work well for one of our cats (must be hard to read for some reason).

Is there anything I can do? My cats are too scared to go out. Both of them were very badly attacked by another cat at our old house, one had her eye badly scratched, and the other literally had her stomach split open and needed surgery, so they are very cautious these days!