GPs opted out of weekend and night care in 2004
There is an unacceptable variation between GP out-of-hours services, the government admits.
But health minister Mike O'Brien said care had still improved since doctors gave up providing cover in 2004.
He was speaking after a report suggested marked differences in levels of cover and home visits.
He said the government would publish its own review of night and weekend care following the conclusion of a high-profile inquest later this week.
A Cambridgeshire coroner is due to give his verdict on Thursday into the deaths of David Gray and Iris Edwards, who had been seen by out-of-hours doctor Dr Daniel Ubani.
Dr Ubani, who is a cosmetic surgeon in Germany, was doing shifts for Take Care Now, the company which was running the NHS out-of-hours service in Cambridgeshire. The contract was terminated at the end of last year.
In April 2009, the German authorities gave Dr Ubani a nine-month prison sentence, suspended for two years, and he was ordered to pay a £4,500 fine for causing death by negligence.
A subsequent report by the Care Quality Commission last year then raised further concerns about out-of-hours care.
The regulator looked at five trusts which used Take Care Now and warned they were failing to properly monitor services.
It said the variations in performance suggested it could be a nationwide problem.
Most GPs opted out of providing out-of-hours care in 2004.
Since then local health managers working for primary care trusts (PCTs) have been responsible for providing cover at nights and weekends and have often relied on private firms working under NHS contracts.
Responding to questioning about differences in services, Mr O'Brien told GMTV the service was not a shambles as services had actually improved over the past six years.
But he added: "There is an unacceptable variation between primary care trusts."
His comments come as figures from the Primary Care Foundation, a research group which compares performance for PCTs, illustrated the scale of the differences.
For example, the number of patients receiving home visits varied from one in four to one in 50.
It also showed that some areas were using one GP to cover a population of 300,000.
But Henry Clay, director of the foundation, agreed it would be wrong to assume the system was in chaos, pointing out services were only receiving a small number of calls on average.
"There are still areas that need addressing, but in many ways it has improved."
Dr Richard Vautrey, chairman of the British Medical Association's GPs committee, said: "There is a problem with consistency. It is down in part to the investment by PCTs. Out-of-hours has been subject to cost-cutting."
But he added: "We can look back with rose-tinted spectacles. Before 2004 GPs were tired because they were working day and night and that was not good for patient safety."
Lib Dem health spokesman Norman Lamb said: "We have inadequate safeguards to stop dangerous doctors from overseas from practising here and threadbare cover in many parts of the country."