The policy of providing free nursery places for three-year-olds has delivered no long-term educational benefits and helped only a small number of women into work, according to two studies.

Tony Blair's Labour administration began offering free part-time pre-school education in England in 1998, and by 2007 some 88% of three-year-olds were benefiting. Labour has promised to extend the offer from 15 to 25 hours if it wins next year's general election, while Liberal Democrats have pledged to make free places available for two-year-olds.

The new studies questioned the value of the £800 million-a-year policy, as they found that it had helped only a relatively small number of women into work and that any boost to children's attainment does not last.

Research by the Institute of Education and the universities of Surrey and Essex found that free nursery places had "a small beneficial impact" on English children's development at the age of five, but the size of the effect declined by seven and disappeared altogether by the time they reached 11.

There was "modest" evidence that the policy had more impact on the poorest and most disadvantaged children, but it did not close the gap in attainment between those from richer and poorer families in the longer term, the researchers found.

A separate study by the Institute for Fiscal Studies and the University of Essex found that for every six children given a free place, only one additional child began to use early education. For the other five children, the policy was effectively giving parents "a discount on the early education they would have paid to use anyway", found the researchers.

The number of mothers whose youngest child was aged three and who were in work increased from 53% to 56% - equating to an additional 12,000 women in work, most of whom were moving into part-time posts of less than 30 hours a week.

The IFS study found that this meant the policy was costing £65,000 for each extra person helped into work, which it described as "a very expensive employment policy".

Mike Brewer, IFS research fellow and professor of economics at the University of Essex, said: "In recent months, Labour, the Liberal Democrats and the Scottish National Party have all promised to spend additional money to extend the free entitlement to early education.

"Our results suggest that the current approach is improving - but by no means transforming - the labour market attachment of mothers of young children. The expansion of free early education in the 2000s was a very expensive way to move an additional 12,000 mothers into the labour force, and the case for extending the free entitlement is not as clear-cut as political rhetoric might suggest.

"A more open and honest debate about the rationale for these policies, and whether the evidence supports these positions, would be welcome."

On the educational benefits of free pre-school places, Jo Blanden, a senior lecturer in economics at the University of Surrey, said: "While previous research has suggested that early education is key to long-term attainment, our research has shown that the free entitlement did not deliver the anticipated gains.

"On the face of it, our results cast some doubt over the value for money of universal early education. More than 80% of the children taking up free places would probably have gone to nursery anyway, and the policy had no educational benefits in the longer term.

"This might be because the extra pre-school places were not of high enough quality. Recent research shows that state-run nurseries are of more consistent quality, whereas this policy encouraged greater use of privately run places. Alternatively, it could be that primary schools do very well at helping children reach their potential, meaning that pre-school experience is not important."

Birgitta Rabe, senior research fellow at the University of Essex's Institute for Economic and Social Research, said: "It is tempting to say that the money would have been better spent on the poorest children. However, the policy's universalism may have benefits if it encourages greater take-up of provision among children from more disadvantaged backgrounds or if it mixes children from different backgrounds in the same early education settings.

"In this context, it might be more appropriate to see the free entitlement as a form of child benefit, and the policy debate should reflect this."

Education and Childcare Minister Sam Gyimah said: "Before they have even worn their school uniform for the first time, a child's life chances are being decided.

"Early education not only sets a child off on the right foot at school but, as the extensive research including the most recent report from EPPSE (Effective Pre-School, Primary & Secondary Education) published last month shows, it has effects that last right into the workplace.

"We know that parents value the early education entitlement which is why 97% of three and four year olds in England - nearly 1.3 million children - are taking up their early education place.

"No child should start school behind their peers. This is why as part of our plan for education we are committed to providing flexible, affordable and good quality childcare, giving parents more of a choice about where they can send their children, so that they can get the best start in life."

Neil Leitch, chief executive of the Pre-school Learning Alliance, said: "It is important to note that these findings suggest that the introduction of free early education places had a limited impact on outcomes, largely because most children accessing places would have done so anyway, even if the places were not free.

"This is not the same as saying that early education itself has a limited impact, and should not be misinterpreted as such.

"In fact, the research found that for those children who otherwise would not have accessed any early education, the provision of places through the free entitlement scheme had a notably positive impact: an increase in Early Years Foundation Stage Profile scores at age five of 17% on average.

"We are surprised, however, by the suggestion that this positive impact becomes less evident in the long term, and it should be noted that this contradicts a wealth of existing research which has found that the opposite is true: most notably the recent EPPSE study which concluded that early years provision has 'enduring benefits' for those children that attend, including better outcomes at GCSE and greater earning potential in the longer term."