The BBC's Aminatta Forna:"The choice mothers have to make is becoming harder"

The Institute of Education studied 17,000 children whose parents were born in 1958.

Results showed children whose mothers worked when they were under a year old had slightly lower reading scores than the average for the group.

But those whose mothers returned to work when they were older became more confident, with better social skills.

Author of the report Heather Joshi: "The decision should be with the mothers"

The research was used by former social security minister Harriet Harman in a meeting with Chancellor Gordon Brown on Wednesday to propose a baby tax credit for mothers of children under one.

She says the tax credit will give them more choice over whether to go back to work or stay at home with their children in the first year of their life.

The study, commissioned by Ms Harman in conjunction with the Smith Institute, measures children's mathematical and literacy skills as well as aggression and anxiety.

Aggression is judged by disobedience, lying, difficulty concentrating, bullying behaviour, restlessness and destructiveness.

The study found no particular difference in the development of children under one whose mothers went out to work, compared with those who stayed at home, except in reading skills.

Harriet Harman: Lost her job but retains strong interest in women's issues

But children aged one to four who had a full-time working mother were significantly less anxious and had slightly better maths skills than those whose mothers did not work.

There was no significant difference for literacy skills and aggression.

Children aged five to 17 were much less likely to be anxious if their mothers worked, particularly if they worked part-time

The report's author Heather Joshi said it showed there was no real difference if a mother went out to work or not.

Commenting on the slight reduction in reading skills for children whose mothers work before they are one, she said: "We imagine that maternal atttention is important as well as the mother's health and a family free from stress, but we cannot tell that from the evidence we have got."

The institute is now assessing the impact of working on mothers.

The study, which confirmed previous research in the US, also found that poverty and a mother's academic ability influenced her child's educational development much more than whether she worked.

Children of unemployed parents fared significantly worse than others and those brought up in social housing were more aggressive and had fewer reading and maths skills than average.

'Increase mother's choice'

Most working women are allowed up to six months off work after the birth of their child, although not all of this may be paid.

Some have to go back to work out of financial necessity only two or three months after their babies are born.

The government, anxious to reduce its benefit burden, is keen to encourage as many women as possible back into work.

Harriet Harman used her Wednesday meeting to try to persuade Mr Brown to support her "mother knows best" policy.

She urged him to bring in a baby tax credit of about £70 a week, similar to the childcare tax credit now available for older children.

Ms Harman also asked Mr Brown to bring in legislation extending the period employers must keep jobs open for pregnant employees from six months to a year.

Currently, mothers only get statutory maternity pay for 18 weeks following the birth of their child, but the proposed tax credit would kick in once that ended.

Jill Kirby of the organisation Full-Time Mothers said: "There are financial incentives to go out to work, but no financial incentives to stay at home."

Ms Harman commented: "What we should be doing is increasing the mother's choice when her baby is in early infancy."