Disadvantaged five- and six-year-olds are catching up with their more affluent classmates when it comes to phonics skills, new data reveals.
But on a broader range of measures, including social, emotional and physical development, poorer pupils are still lagging behind.
Primary school leaders warned that the latest figures from the Department for Education showed that more work was needed to help Year 1 pupils eligible for free school meals to close the gap with their peers.
In 2013-14, 61.3 per cent of pupils eligible for free school meals met the expected standard in the Year 1 phonics check - 15.7 percentage points lower than the figure for more advantaged students. The attainment gap dropped from 16.6 percentage points in 2012-13, which is three times the decrease recorded during the preceding 12 months.
However a separate government measure, which takes into account a much wider range of factors, shows that little progress has been made in closing the gap for FSM pupils.
The "good level of development" measure assesses literacy, numeracy, physical development, communication, language and personal, social and emotional development among five-year-olds.
Based on this standard, the attainment gap between deprived children and their more affluent classmates fell by just 0.1 percentage points between 2012-13 and 2013-14.
The figures have been released ahead of the introduction of the early years pupil premium in April, which is intended to tackle disadvantage among preschool children. Under the policy, early years providers will be able to claim an extra 53p per hour for every qualifying three- and four-year-old in their care.
At Water Hall Primary School in Milton Keynes, about 60 per cent of pupils are eligible for free school meals. Headteacher Tony Draper said that many pupils arrived at the school with a "very low level of development".
"A lot of children don't have a good level of language acquisition before they get here, or social skills," he added. "As an overall cohort, they're entering school at a very low level in comparison to affluent areas."
Mr Draper believes the national improvement in phonics scores has been triggered, in part, by Ofsted's focus on the measure. "With schools being condemned as a result of low phonics screening scores, they are getting better at teaching phonics," he said.
But he also warned that the attainment gap in terms of pupils' wider development was unlikely to close in the short term.
"It will take the whole of the primary-age group to narrow the gap. It doesn't happen overnight. And if the government wanted it to happen overnight, they'd be in cloud cuckoo land."
Russell Hobby, general secretary of the NAHT headteachers' union, said the improvement in phonics attainment resulted from a greater focus on disadvantaged pupils in primaries.
"I think there is definitely a more systematic use of phonics now and that is going to explain part of [the improvement], but also there is more targeted one-to-one catch-up work and smaller groups with children at risk of falling behind," he said. "The pupil premium has given schools the money to invest in the strategies that work.but as with all initiatives it will take time to filter through."
A DfE spokesman said the first few years of a child's life were "make or break in terms of how well they go on to do at school and beyond".
He added: "These statistics clearly show that progress is being made. But we will continue to work to ensure that all children, especially those from disadvantaged backgrounds, are given the skills they need to succeed."
A House of Lords report published this week by the committee on affordable childcare calls for an urgent review of spending on early education and childcare, and for more funding to be targeted at disadvantaged children.
"For this group in particular, the impact can be substantial," said the committee's chair, Lord Sutherland of Houndwood.