Only a third of the children in poorer areas of England are showing a good level of development after a year in school, official figures reveal.
Boys tend to develop less quickly at first than girls
This compares with half the children in the rest of the country, as measured by their Foundation Stage Profiles.
There was no progress in 2006 on the government's target of closing the gap.
The latest analysis follows initial statistics for 2006 showing fewer children than previously thought could write their names or read simple words.
The key assessments cover personal, social and emotional development and communication skills among children at the end of their first year of schooling.
The figure for the 30% most disadvantaged areas was that 33% of pupils were said to be working securely within what are known as the early learning goals.
For the rest of England it was 50%, according to the statistics published by the Department for Education and Skills (DfES) - a gap of 17 percentage points.
The equivalent figures for the previous year were 37% and 54%.
The average for the whole of England fell from 48% in 2005 to 44%.
Ministers and officials had said the apparent falls between 2005 and 2006 appeared to be a result of more accurate assessments being made, within what is still a relatively new system.
In its spending review white paper in 2004, the government announced two targets to be met by 2008:
- 53% of children should reach a good level of development
- reduce inequalities between the poorest areas and the rest of England to 12 percentage points
A spokesperson for the DfES said: "We remain fully committed to narrowing gaps in achievement.
"That is why we have put measures in place through the Childcare Act, including a specific duty on local authorities to secure improvements in outcomes and reducing gaps in achievement.
"Measures like these will help to ensure a greater focus is placed on young children's outcomes."
Shadow children's minister Anne McIntosh said the figures demonstrated that Labour was failing to make any ground in helping the poorest pupils in society.
"At this crucial early stage, pupils from deprived backgrounds are falling even further behind," she said.