According to the National Children's Bureau report Leaving care: time for change, not enough is being done in council education departments to inform care-leavers of educational grants for which they may be eligible.
The findings - based on an audit of the capital's local authorities - were consistent with the views of the young people themselves, the report says.
Many young care-leavers felt they were given inadequate financial support in further and higher education. They also felt council social services departments gave low priority to their educational needs and thought they received inadequate careers guidance.
The six-month National Children's Bureau study was published jointly by the Government's Rough Sleepers Unit and the Department of Health.
It analysed the services for those in care in all 33 London boroughs through a series of interviews with public- and voluntary-sector professionals and with young people who had been through the care system.
Few authorities had an overall strategy on consulting young people in their care. Many ceased to look after young people when they reached he age of 16 or 17. Leavers were often allowed to drift away without being formally discharged.
The Department of Health estimates one in five care-leavers experiences a homeless period within two years of leaving care.
The report follows the unveiling this week of government grants totalling more than pound;500,000 to fund new projects to prevent young people across the country ending up homeless.
Among the new projects are a scheme, based in Oxford and Brighton, where excluded pupils will be educated about the dangers of sleeping rough and an initiative in Newcastle, where former homeless young people will go into schools and explain to pupils how to prepare for the challenges of independent living.
The Government has introduced a raft of measures to tackle the problems of care-leavers. In particular, the Children (Leaving Care) Bill, now going through Parliament, will considerably extend local authorities' duties to support them.
A Government spokesman said: "The audit tells us much of what we know already, that this area is in transition." He added: "Much remains to be done to bring services up to scratch."
For copies of "Leaving Care: time for change," ring the Rough Sleepers Unit on 020 7944 3600.