The government appears to be paying out benefits and tax credits to 200,000 more lone parents than exist in the UK, a think tank study suggests.
The system is aimed at easing the burden on single parents
The Institute for Fiscal Studies says there are 1.9m lone parents in the country, yet 2.1m get government money.
It suggests many may be exploiting vagueness on the definition of a lone parent - co-habiting with a partner but describing themselves as single.
The government rejected the findings and said the study was misleading.
It said the system rightly allows people who might be in loose cohabiting relationships to call themselves single.