The investigation, based on data obtained under the Freedom of Information Act, also showed 35% of centres had registered fewer than 1,000 patients, prompting calls from GP leaders for the centres to be shut down. The story was also covered by The Independent, Daily Telegraph, Metro and BBC Radio London.
The category winners of the MJA Winter Awards will be announced at an award ceremony in London on 25 January.
Last year GP news reporter Stephen Robinson was the overall winner in the medical journalist of the year category of the MJA Summer Awards.
GP columnist Dr Liam Farrell also won the best health or medical column for his work on GP at the Medical Journalists' Association Winter Awards in 2011.