The introduction of a computer system to track immunisations in London has been delayed, despite fears of a national measles epidemic.
It is due to replace a system which failed to give accurate figures on how many children had the MMR vaccine.
But supplier BT said the "complex" new system would not be ready until mid- 2009, six months later than planned.
The government has begun a campaign to raise MMR vaccination rates in England, amid concern about a measles epidemic.
The MMR jab is first given at about 13 months of age and the child is then given a pre-school booster.
Inaccurate data
Last month the NHS reported that fewer than half of all five-year-olds in London have had the MMR vaccine and booster jab.
But the computer system used by NHS London to manage the capital's vaccination programme - known as the Child Health Interim Application (CHIA) - was found to provide inaccurate data.
Introduced three years ago, CHIA also failed to send out appointments or reminders to patients as required.
BT is developing an improved alternative to CHIA entitled Rio, which was due to be in place by January 2009.
But a BT spokesman told BBC London that Rio was still being developed and its introduction would be delayed for six months.
"It will be difficult to give an absolute guarantee to 'go live' dates as these are complex systems," he added.
"However we have an agreed deployment schedule which is reviewed by all parties on a weekly basis and as of today we are confident of meeting it."
The spokesman added: "We recognise that there have been some issues with CHIA, but there is no proven link between CHIA and low immunisation rates."
A spokesman for NHS London said BT would not be paid for the computer system until it was found to be effective.
BT has a £996m IT contract with NHS London. It would not reveal how much it cost to develop the vaccination management systems.
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