More babies are born at dangerously low birth weights in Britain now than in 1989, a report says.
Low birth weight is linked to many problems
The study was carried out by the Fabian Society, a left-leaning think-tank, which called the finding a "scar on the national conscience".
It calls for more financial support for at-risk women, better access to antenatal services and one-to-one care for all newborns in intensive care.
A minister said the recommendations must be studied carefully.
The researchers found that in 2006, 78 out of every 1,000 babies were born weighing less than 5lb8oz (2.5kg). That amounted to a total of more than 50,000 babies.
In 1989, 67 out of every 1,000 babies were born under weight.
Low-birth weight is linked to an increased risk death and disability, and a range of long-term health problems, such as diabetes, heart disease, Attention Deficit Hyperactivity Disorder (ADHD) and depression.
The report found that older women and teenagers were most likely to have a low birth weight baby.
It said more work was needed to cut teenage pregnancy rates, and employers should offer more support to women, so fewer felt compelled to put off having a child.
The report also found lone parents were nine times as likely to have a stillbirth as other parents.
Babies born to working-class mothers were twice as likely to die before their first birthday as those with middle-class parents.
Mothers of Indian, Pakistani and Bangladeshi origin had a high risk of having low birth-weight babies - their babies are on average 300g lighter than those of white mothers.
These mothers also attended fewer antenatal appointments than other ethnic groups.
Lead researcher Louise Bamfield conceded that medical advances meant more low birth weight babies were now surviving, but she said social deprivation and low income were significant factors.
She said: "If Britain had the same record on low birth weight as the best countries in Europe, 24,000 babies would have much improved life chances.
LOW BIRTH WEIGHT
"The facts should shock us all. Britain has the worst rate of every country in western Europe, except Greece.
"And being born very small creates health risks throughout life - and will affect the health of babies they will themselves have years later."
Constitutional Affairs Minister Harriet Harman said: "We have made important progress since 1997 on reducing child poverty and creating the early years agenda.
"We must now be bolder and develop the new policies to make building a fairer society the central theme of Labour's next term in office.
"The Fabian Society's evidence on inequalities at birth must be studied carefully by government and the Labour party. The political argument will need to be won too."
Andy Cole, chief executive of Bliss, the premature baby charity, said: "Around 40,000 babies are born underweight each year in England alone, and many will undoubtedly spend time in a neonatal unit before going home.
"The very first days after birth are critical for their health and it is vital that the recommended standard of one-to-one nursing for babies in intensive care is made mandatory."