The Health Protection Agency (HPA) has warned that tens of thousands of children in London could contract measles due to low immunisation rates.
Roger Gross, the local HPA director, told BBC London there was a "very real risk of a large outbreak" which could leave some "permanently disabled".
The city registered the highest number of cases in a decade. Hackney in east London has been the worst-affected.
There are also concerns about whether a new IT system is tracking cases.
'Permanently disabled'
Since June, City and Hackney Primary Care Trust (CHPCT) reported 297 cases compared with their yearly average of between 10 and 20 cases.
Dr Gross, said: "I think because of the underlying low vaccination rates through the whole of London there is a very real risk of a large outbreak.
"When I say large, I'm talking of tens of thousands of cases, in which case sadly there will be some children who would be permanently disabled by it," he said.
The number of children receiving the measles vaccine fell after research - since discredited - suggested a link between MMR and autism.
Doctors are also concerned that a new IT system installed two years ago in 10 out of London's 31 primary care trusts to manage the vaccine programme has not been able to provide accurate data.
The system is called Child Health Interim Alert (Chia).
Dr Mike Fitzpatrick, a GP with CHPCT for 20 years, said: "The IT system for immunisation has been one in the list of NHS IT fiasco.
"Its true that at the moment in the whole of the north Thames area the authorities have only a very vague idea of what the level of uptake is because of the problems with the computer system."
But Dr Gross said the faulty computer system was not entirely to blame for the high number of measles cases.
Dr David Elliman, who is part of a group looking after Chia, said: "There is research that shows that good appointing systems do make a difference. Currently we don't have a good appointing system," he said.
In response to high measles cases CHPCT launched a special on-the-spot immunisation programme, called spotty bus, targeting schools and nurseries.
A new computer system is expected to be in place in the next 14 months.
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