Middle-class children who have been to playgroup score higher in reading tests than their peers who have attended nursery school, new research has revealed.
It discloses that children from all social groups who have attended playgroup do better at writing, score exactly the same as nursery pupils in number tests, and perform only slightly worse at science.
The research by Sandra Daniels from the University of Leeds comes as ministers decide whether to find an estimated Pounds 50 million to fund full vouchers at playgroups.
The pre-school playgroup movement has threatened to boycott the Government's Pounds 730 million nursery initiative in protest at ministers' decision to give them only half the value of the Pounds 1,100 voucher.
According to the research, which is based on analysis of pre-school experiences of 1,800 pupils, and teacher assessment and test results for seven-year-olds, both nursery schools and playgroups have a positive effect on children's achievement compared to non-attendance.
The children with nursery experience came predominantly from the lower socio-economic groups, while the children attending playgroup tended to be from the higher social groups.
It reveals that 91 per cent of children from the higher social groups who attended playgroup achieved a level 2 or higher at reading compared to 85 per cent of children in the same social group who went to nursery.
But while the playgroup had the most impact on reading scores for the higher social groups, for the lower social groups performance was worse than the non-attenders.
In the lower social groups, 73 per cent of nursery children achieved level 2 or more in reading, compared to 61 per cent in playgroup and 66 per cent of children who had been to neither.
When all social groups were taken together, the non-attenders produced the lowest scores in reading, writing, number and science tests.
In reading and science, the nursery children appeared to produce the higher average scores, while the playgroup children had the higher for writing. In number work, the two groups showed no difference.
Sandra Daniels said that in none of the subjects were the average scores for playgroup and nursery children significantly different.
She added: "There was no evidence that the outcomes of the national curriculum assessments of nursery and playgroup children were significantly different from each other, just that either was preferable to none."
But her findings, published in last month's Oxford Review of Education, are certain to be seized upon by the pre-school playgroup movement, as ministers have not yet answered their complaint that they get only Pounds 550 a year for providing a morning or afternoon place every weekday.