Ensuring adults working in early-years settings have good qualifications and ongoing training is key to raising the quality of provision, a study suggests.
Researchers at Oxford University found well trained staff could make up for larger numbers of children to adults.
But they stress that this is not an argument to reduce the ratio of staff members to children, which should be "maintained at a favourable level".
They also suggest the quality of care in England has risen in recent years.
The Oxford researchers analysed data from 598 early childhood education and care (ECEC) settings across England - a mixture of private and voluntary providers, children's centres and nursery schools.
They collected information on the training, qualifications and staff-to-child ratios of early-years workers looking after three- and four-year-olds who were born between September 2010 and August 2013.
Their analysis found that state-funded settings tended to have higher quality ratings and suggest the presence of highly qualified staff maintained this quality, despite the fact that staff to pre-schooler ratios were less favourable.
Lead researcher, Prof Edward Melhuish, said: "A better staff-to-child ratio leads to improvements in quality but staff qualifications and training is the most important factor.
"Our study shows that having well trained and qualified staff increases the quality of education and care in a child's early years.
"Also, better staff-to-child ratios mean staff can spend more time in one-to-one interaction with children and this is very beneficial."
The study concludes: "Countries wishing to improve the quality of their ECEC provision should actively seek to improve both staff qualifications and in-service professional development."
It points to recent research in Australia indicating in-service professional development had "clear effects upon observed quality in ECEC as well as potential effects for child outcomes".
The researchers call on policy-makers to work to "increase staff qualifications and to provide enhanced opportunities for ECEC staff to obtain in-service professional development".
"Additionally, staff-child ratios should be maintained at as favourable a level as is pragmatically viable," the study says.
Improvements in care quality
The study also compared the data - gathered in 2014-16 - with information from a different study of pre-school children in England conducted in 1998 and 1999, to assess the impact of "a period of extensive policy change".
It found that the quality of early-years education had "risen significantly" in that time frame.
Prof Melhuish said: "While there is still a long way to go, the evidence suggests that the policy changes in the UK have led to higher-quality early childhood education and care.
"Existing evidence would lead us to expect that these changes will have long-term benefits for the population and future economic development of the country, as economic development in the modern world is increasingly dependent on the education of the workforce."
The research paper is published in Frontiers in Education.