At Labour's annual conference in Manchester this week, Mr Johnson also pledged to pay an extra pound;100 into young people's trust funds for every year they are in care - money which they can spend when they reach age 18.
But a spokesperson for the Assembly government said that over the past five years a lot had already been done at national and local level, including a pound;1,800 remission grant for looked-after children going into higher education.
"We already announced back in June that, from this month, we will be reimbursing local authorities for each annual pound;50 payment to the children trust funds for looked-after children," added the spokesperson.
England's record on looking after children in care has been criticised by one of Tony Blair's former advisers. Writing in today's TES, Julian Le Grand, of the London School of Economics Centre for Analysis of Social Exclusion, says that targets are damaging the achievements of children in care because they encourage schools to ignore low achievers.
Gordon Brown, the Chancellor, admitted on Monday that children in care had been one of the Government's "greatest failures of social policy".
Mr Johnson said: "No one suffers greater disadvantage in our education system than children in care. They are five times less likely to pass GCSEs and 24 times more likely to wind up in prison. Some of these children escape the most terrible abuse and neglect in their own family, only to experience the chill of indifference of their proxy parent, the state.
"Instead of bringing them up we let them down, passing them from one location to the next, dumping them in the worst schools and forcing them to fend for themselves from the age of 16.
"Next month we will publish wide-ranging proposals to tip the scales in favour of children in care making the state a good parent."
In the TES article, co-written with David Chater, head of policy and external affairs at young people's charity Rainer, Professor Le Grand calls for schools to be given extra money to provide support for children in care.
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