Animal rescue centres in West Norfolk say they facing a crisis due to an influx of kittens following a cat population explosion.
Shelter staff said they have been forced to turn away kittens because all of the pens are full.
Three litters have been dumped in separate cardboard boxes in different parts of the borough in the last month.
The population problem has been put down to owners failing to have their cats neutered.
Staff at the Cats Protection League shelter in Stow Bridge are caring for 25 kittens.
Manager Lindsay Tempest said: “We are at capacity with cats. We only have a limited number of pens. We are getting to crisis point.
“We don’t have any space at all unless people come and rehome from us. We can’t take any more.
“It is heartbreaking when someone calls and you can’t help them as you don’t have any where to put them.
“The message is still not getting out there about how important it is to have cats neutered. Most of the kittens we have come from owners and not strays.”
It is a similar situation at the RSPCA Rehoming Centre in Eau Brink, which is now caring for 30 kittens.
Manager Penny Skate says numerous messages are also being left daily on the centre’s answerphone from desperate owners.
Mrs Skate said: “We have an over population of cats as it is such a popular pet.
“We are completely full and can’t take any more cats or kittens. It is particularly busy with more and more people bringing them in.
“In the last 10 days we have been inundated with large amounts of cats and kittens. They often bring the mum as well as the kittens.”
RSPCA inspectors have brought in three litters of kittens which have been dumped.
Three kittens, a ginger, tabby and tortoiseshell, were found in woodland around Lynn on July 2. These are believed to be a month old and were not feral.
Another box was found containing two kittens, which are tabby and black, two weeks ago.
Inspectors also found four kittens in another box during the last fortnight.
Mrs Skate said: “People will just leave them as they don’t know what to do with them.”
She is also stressing the need to have cats neutered.
Mrs Skate said: “It is important for people to realise the importance of neutering cats and this looks like it is going to be a big summer for kittens.
“As a species we are rather over populated at the moment and there are people out there letting their cats have kittens – I don’t know why they are!
“It is so much better to have them neutered and there is help available for people on low incomes.”
Nationally, the number of cats entering RSPCA care increased from 29,l269 in 2010 to 31,556 in 2012. In the same period, the number of cats being rehomed fell by 10 per cent.
The charity is also relying on private catteries as its own pens are full. The cost of boarding went up from £1.9 million in 2010 to £2.45 million in 2013.
Owners are advised to have their cats neutered at four months. Up to 85 per cent of litters are unplanned.
The operations can cost about £45 to spay a female and £31.25 to castrate a male.