Fewer than half of nurseries are likely to extend free childcare places for pre-schoolers to 30 hours a week, amid major concerns about funding, research suggests.

Under Government plans, free childcare and early-years education for three and four-year-olds in England is to be doubled from 15 to 30 hours for each week of the school year, a move ministers say will help support families. Pilots of the scheme start this autumn.

But while many nurseries are keen to help parents by offering the full 30 hours, there are fears that government funding to provide the places will not be enough to cover their cost, according to research by the National Day Nurseries Association (NDNA).

A survey conducted by the association found that 45% of the nurseries polled say they are likely to extend the number of free hours on offer.

At the moment, nurseries are managing to offer 15 hours of free childcare a week because they plug the shortfall in government funding, the NDNA's report said, adding that in practice, this means that parents pay a higher rate for the hours their child spends in nursery above 15 hours.

The average nursery has to absorb around £34,000 a year due to the funding gap, with 89% of nurseries making a loss on free places.

"Doubling this entitlement to 30 funded hours for working parents of three and four-year-olds is going to increase this shortfall further but leave nurseries with no means of making up that loss," the report says.

The NDNA said its survey had shown that only a third of youngsters aged three and four attend nursery for more than 30 hours a week.

NDNA chief executive, Purnima Tanuku, said: "The nursery sector is fully behind the principle of more support for parents. But serious funding shortfalls stand in the way of nurseries getting on board, despite their desire to help families with free childcare.

"Private, voluntary and independent nurseries deliver most of the Government's free places, currently 15 hours per week for all three and four-year-olds and some two-year-olds.

"But the nursery sector is reluctant to commit to offering more free hours when they already make a significant annual loss - an average of £34,000 per nursery - on the funded places they currently provide."

She added: "The nursery sector and the Government want the same thing. We all want to make 30 free hours childcare workable and sustainable.

"To achieve this, the sums have to add up. Urgent steps must now be taken to bring about the funding reform promised by the Government, so an economically viable hourly rate for high-quality childcare reaches the front line."

Education and Childcare Minister Sam Gyimah said: "The childcare sector is healthy, vibrant and growing. More parents than ever are accessing high-quality free childcare and thanks to our plan to double the offer for working parents, we expect that trend to continue.

"We are backing families and funding the sector, with £1 billion extra funding every year by 2020, including £300 million annually to increase the national average funding rate, to incentivise and attract providers to deliver the full 30-hour free offer to parents.

"This extra funding was based on an extensive consultation with the sector and our review into the cost of delivering childcare, the most comprehensive analysis of this market ever.

"During the last Parliament the sector demonstrated its ability to grow with the extension of the free entitlement to disadvantaged two-year-olds and the creation of 230,000 more places since 2009."

Shadow childcare minister Jenny Chapman said : "The Government's childcare plans are in a complete mess.

"One in three working parents promised free childcare by David Cameron at the last election will miss out because Tory ministers are unable to make the funding for their manifesto childcare pledge add up.

"This further news is a body blow for parents struggling with the rising cost of childcare as nurseries plan to turn them away.

"Ministers have no plan to improve the quality or sustainability of childcare. The Tories' pre-election rhetoric on childcare has failed to match reality and parents, children and our economy will pay the price."