‘Accidental’ litters contribute to population growth and the number of unwanted animals entering animal welfare organisations. Assessing the problem's extent and determining risk factors enables identification of education targets. Data were obtained from 715 cat-owning households in a cross-sectional telephone survey. Demographic and lifestyle factors were assessed for their association with accidental litters and with owner knowledge of cat reproduction. A total of 128 litters were reported from 552 female cats, and the proportion of accidental litters reported by owners was 80 per cent. Multivariable analysis identified that respondents were more likely to report an accidental litter of kittens if they believed a female cat should have a litter prior to being neutered, if they had more than one cat and if they rented rather than owned their home. Misconceptions relating to cat reproduction were common. The opinion that the youngest age a cat could get pregnant was five months of age (or older) was held by 83.5 per cent of cat-owning respondents, with over a quarter (26.4 per cent; 174/659) believing a queen is unable to conceive until at least a year of age. Almost half the respondents (49.0 per cent; 334/682) believed a female cat should have a litter before being neutered or were not sure; 38.8 per cent (264/681) thought that un-neutered, related cats would not mate or were not sure. This study suggests that improving cat-owner knowledge of the reproductive capacity of cats is likely to have a significant impact on the numbers of accidental litters born.