Married couples living together are suffering under the tax and benefit system which favours parents who have separated.
By staying together, couples could lose between £58 and £68 every week.
The figures come in a report by CARE (Christian Action, Research and Education) which has released studies on the ‘couple penalty’ for the last three years.
Nola Leach, CARE’s chief executive, said: “It is very disappointing to see that, far from being eroded, the number of families negatively impacted by the couple penalty has actually increased for the third year in a row.
“Rather than using taxpayers’ money to create fiscal incentives that make it more likely that children will grow up in a home with only one resident parent, we should at the very least ensure that parents are not disadvantaged by living together under the same roof with their children”, she added.
The report distinguished between couples where maintenance was and was not paid. In cases where it was paid, 74 out of 98 families were £58 a week better off for being separated.
When maintenance was not paid, some couples could be £68 worse off for staying together.
Iain Duncan Smith MP, chairman of the Centre for Social Justice (CSJ), said: “I am pleased that CARE has produced this important research. The need to tackle the couple penalty clearly remains an important priority.”
At the beginning of July, the CSJ recommended a strategy to reverse family breakdown which would give tax breaks for married couples and make divorce harder.
The authors of the CSJ report said marriage was of “paramount importance to individuals, children, communities and our nation”.