Family doctors are earning thousands of pounds working night and weekend shifts at stretched A&E units, it was reported.
GPs are being paid up to £1,500 a shift as they help crisis-hit casualty departments cope with soaring numbers of patients, the Daily Mail said.
Four out of 10 accident and emergency departments are hiring family doctors, who are already paid an average of £104,000 a year.
Some 86 of the 130 NHS hospital trusts in England with A&E units responded to a Freedom of Information request by the newspaper asking if they employed GPs in their casualty departments.
Of those, 36 said they hired GPs in roles that included assessing patients as they arrive, treating the less seriously ill and providing general cover.
The figures emerged after a poll found that a third of GPs believe patients should be charged up to £10 a time for some A&E visits to help cut unnecessary attendances.
Pay levels for family doctors working in casualty wards ranged from £50 an hour at Wirral hospital in Merseyside to £100 an hour at Portsmouth Hospitals, the Mail said.
One recruitment firm, Primary Care People, was reported to offer family doctors £120 an hour for a 12-hour shift - £1,440 - at Guy's Hospital in south London.
Pay rates through the company were so good that doctors are reportedly travelling to the capital from all over the country to make the extra money.
For the last decade GPs have been allowed to choose not to work out-of-hours, fuelling a 50% increase in patients going to casualty.
Roger Goss, of Patient Concern, told the Mail: "GPs can't be prepared to turn out for their own patients but they are happy to work elsewhere for a lot of money.
"So much for the interests of the patients, it's the interests of the doctors which are paramount. The reason patients are going to A&E is because they can't get into their own surgery. It's ludicrous."
Conservative MP Sarah Wollaston, a GP who is on the Health Select Committee, said recruiting GPs to work in casualty units could result in even more patients turning up as they realise they can see a doctor more easily.
A Department of Health spokeswoman said: "It's not unusual for A&E departments to employ in-house GPs. They provide a valuable source of expertise and help patients to be seen and treated promptly.
"There are over 20 per cent more A&E consultants now than there were in 2010, but we know more are needed, so we've set up the first ever independent body to work on recruiting more doctors to A&E and planning extra training posts.
"And we know that the whole system of out of hours care needs to be changed. That's why we've agreed a new GP contract, backed by doctors, which is part of a longer-term plan to bring back the personal link with patients so GPs can focus on giving people the care they need and preventing unnecessary trips to A&E in the first place."