Mr Carle said: "A member of the public was using the recycling bins at around 7.30am when they spotted a cat carrier on the ground.
"Morris and Sonny were inside, but there was no food or water, only some cat litter which was quite dirty. Judging by the conditions inside the carrier I think the kittens may have been left there overnight.
"They would have been extremely cold as the temperature has been dropping significantly lately. Morris and Sonny are in good condition and well handled so they've clearly been cared for up until now."
The charity is keen to trace whoever abandoned the kittens and appealed for anyone with information to call its Animal Helpline on 03000 999 999.
It warned that abandoning an animal is an offence and that anyone found guilty of doing so can face a ban on keeping animals for a fixed period or life.