England's CMO Professor Liam Donaldson announced last week that GP practices would start to vaccinate under-fives as early as December, following the recommendation of the Joint Committee on Vaccination and Immunisation (JCVI).
But negotiating a pay mechanism for the extra workload, combined with existing vaccinations for priority groups means the expansion will be delayed until the New Year at least, said GPC negotiators.
Some practices have yet to receive their first batch of swine flu vaccine, said GPC flu lead Dr Peter Holden.
'Ministers are rushing ahead with this when most practices still have the priority groups to do,' he said.
'GPs cannot set up vaccination clinics until they have the vaccine in hand. What we must not do is set up clinics and then cancel them, as this impacts on public confidence.'
Ian Dalton, the DoH's national director for flu resilience, expects vaccine supply to improve sharply in December.
Adding children to the swine flu vaccination campaign is likely to raise practice work- load by at least 50 per cent. There are 9.3 million people in the existing at-risk groups, and 2.7 million children under five.
But current JCVI advice calls for two jabs for children, while just one is required for adults.
Dr Holden added that GPs would find it harder to vaccinate children. 'We should not underestimate the effort required to get the under-fives in. There has been a lot of terribly unhelpful information on the web that could mislead parents.
'GPs also face the added workload of having to give the under-fives two jabs and the issues of call and recall.'
The DoH and the GPC are negotiating payment and vaccination targets, which GPC chairman Dr Laurence Buckman said 'is not something that can happen overnight'.
Existing targets for uptake of the vaccine in priority groups, set as part of the swine flu directed enhanced service, would be 'irrelevant' when vaccinating the UK's 2.7 million under-fives, and a different payment system may be needed, he said.
It would be 'impossible' for practices to run simultaneous vaccination programmes for the original priority groups and under-fives, he added.
Dr George Kassianos, RCGP immunisation spokesman, said the success of the vaccination programme depends on GPs convincing parents to immunise their children. He called for the scheme to expand to under-18s as early as supplies would allow.