The government's flagship childcare policy, for working parents of three- and four-year-olds, was launched in September 2017.
However, a DfE survey of parents has found that those on low incomes are far less likely to be aware of the offer than more affluent families.
The data shows that 89 per cent of families with an income of £45,000 are aware of the entitlement.
In contrast, just 68 per cent of families with an annual income of less than £10,000 and 67 per cent of those earning between £10,000 and £19,999 a year are aware of the offer.
A key aim of the offer is to help parents with young children into work and take on more hours.
However, the figures show that only 59 per cent of out-of-work couples are aware of the offer, compared to 86 per cent of couples where both work.
Pre-school Learning Alliance chief executive Neil Leitch said it is "worrying" that awareness among deprived families is so low "given studies have shown that current government childcare policies tend to benefit wealthier families over and above those more in need of support".
Stella Ziolkowski, National Day Nurseries Association director of quality and training, added: "It's a grave concern that government policy is not supporting children from low income families equally.
"Fewer children from deprived areas and in families on lower incomes attend nursery. And yet 84 per cent of parents interviewed said the 30 hours had prepared their child better for school, with positive outcomes for communications, numeracy and literacy skills.
"The government must make the funded entitlement available to children who most need it."
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The 30-hour offer is available to working parents with a combined income of up to £200,000 a year.
A study by the Family and Childcare Trust earlier this year found that couples and middle to high earners benefit the most from free childcare initiatives, while those on low incomes and single parents benefit the least.
The House of Commons education committee also raised concerns that the free 30 hours offer was unfairly benefitting affluent families.
However, at a committee hearing in July, children's minister Nadhim Zahawi dismissed such concerns as "faux outrage".
The government data also reveals that parents are most likely to have heard about the 30-hour offer from friends or relatives. This was cited by 35 per cent of those surveyed.
In comparison, 18 per cent were aware of the offer through their childcare provider, while only three per cent found out through their local council or family information service.