Britain's tax system is acting as a barrier to marriage for lower income parents, the Marriage Foundation has said.
The warning comes in response to latest figures from the Office for National Statistics showing that married parents are more likely to belong to higher income groups, with three-quarters (76 per cent) of mothers in managerial or professional positions being married.
By contrast, less than half of new mothers under the age of 35 in middle and lower income groups were married.
Harry Benson, research director at the Marriage Foundation UK, said Britain's tax system was a factor in lower income couples deciding not to get married.
'Alas government policy has actively encouraged this divide since the introduction of tax credits in 2004 which is based on combined household income,' he said.
'The infamous "couple penalty" makes lower income couples thousands of pounds per year worse off if they live together. It is clearly hard to conceal this fact if the couple is married.'
By not marrying, he said the less well off also faced a higher risk of splitting up. Although he welcomed the ONS figures showing that 85 per cent of parents who welcomed a child in 2017 were living together, he alluded to previous research by the Marriage Foundation showing that the odds of staying together were the greatest among couples who married before having children.
In its 2015 study of 1,783 mothers with 14- and 15-year-old children, the Marriage Foundation found that a quarter (24 per cent) of those who had been married at the time of their birth had later split up with their partner.
The proportion of breakups rose dramatically to 69 per cent among those who were not married at the time of having children.
'Being married when your child is born is the single biggest indicator that you will stay together as a couple until your child finishes school,' Benson said.
He further warned that the current system was acting as a 'perverse bribe' by encouraging thousands of couples to pretend they were living apart in order to claim lone parent tax credits.
He said that the problem would only increase under universal credit.
'The more money the state gives out that depends on household income, the more there is to lose when household income includes whatever your spouse or partner earns,' he said.
He added that the government needed to 'wake up to the reality that its welfare policies unintentionally penalise formal commitment among the poorest, make family breakdown pay, and thus thoroughly undermine commitment and stability'.
