First of all whizz them off to vets for a check up to ensure they don't have any UTIs or similar though I suspect this is more behavioural and the fact it has only been in the last three weeks makes it more so. Can you think of anything that may have triggered this? Have you maybe decorated, or had new furniture or had any other pets in the house or moved the tray?

Couple of things to consider.

Is the litter tray big enough? And where is it sited? Ideally is should be in a quiet out of the way place. Is it an open tray? You should perhaps consider getting an additional tray (some cats don't like to wee and poo in the same one!). If you get another tray try a hooded one.

The other thing is that they may not like the litter. If you get a second tray try a different litter. If you are using clumping - then try non-clumping or wood pellets or whatever.

Also consider investing in a Feliway plug in diffuser as if one of them is anxious about something then it may help them.

Good luck.

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Kaz

For kittens, joy is for the asking and smiles are for the giving