A patients' group in Leicester says the city needs a dozen more GPs to provide enough places for people looking for a place at a local surgery.
More than 18,000 in Leicester are not registered with a doctor, figures from the city's primary care trust show.
The Leicester Patients' Group said too many doctors' surgeries were full, which was an "unacceptable" situation.
Leicester City PCT said three new GP surgeries were being built in the city as part of a £3.5m investment.
Patients' group spokesman Zufar Haq said the lack of GPs was putting a strain on the city's accident and emergency department.
"I think it is more than 18,000 - that is just a rough estimate of the people who do not have a GP," he said.
"It is not easy to get a GP to take you on nowadays."
Encouraged to register
He said there was an average of 2,500 patients for each GP in Leicester, but in Derby the average was 1,500 per GP.
Leicester City PCT said it encouraged people to register with a GP.
The national target is 1,800 patients for each GP, so the city needed to add at least 10 or 12 more GPs across the city, Mr Haq said.
There are 63 GP practices in Leicester with about 200 GPs, but areas such as Beaumont Leys, Mowmacre and the city centre have been identified as under-resourced.
Toby Sanders, director of primary care for Leicester City PCT, has said more money needs to be put into health care services where GP lists are already full or not accessible.
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