Doctors have lost more than 300 patients who failed to re-register with a new GP after a practice closed.
Now Oxfordshire health chiefs warn people could miss vital appointments for cervical and breast screening and other routine tests unless they re-register soon.
And they are concerned that elderly patients of Didcot GP Don Langridge, who retired in July, may not get flu jabs this winter.
Barry Coward, business manager at the town's new Oak Tree Centre which has taken on the largest number of Dr Langridge's patients, said: "We know there are young children whose families have not yet registered.
"We are particularly concerned children may miss important immunisations and checks."
He said people were ringing up the surgery expecting to be able to book appointments, only to find they were not actually registered.
People had been given appointments, but because they were not registered, their new GPs could not access their medical records.
By re-registering, patients would also have a number to call out-of-hours if they needed a doctor urgently.
Health chiefs are also worried that the situation will hit local practices' budgets, under new regulations coming into force next year. Dr Tom Jones, Oxfordshire Health Authority's primary care medical advisor, said drugs budgets would be based on patient numbers instead of local population figures from April.
And he warned that the fact that so many Didcot people were now not registered with a GP could make the town a disadvantaged area.
More than 2,000 people were registered at Dr Langridge's King Alfred Drive practice.
People who have not yet re-registered are being advised to contact the Oak Tree health centre because other practices in the town are already full and have virtually closed their books to new patients.
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