The union claimed that proposals under the Health and Social Care Bill, which will see GPs responsible for £80 billion of the NHS budget, GPs "will be free to set their own incomes".
With average GP pay already over £100,000, Rehana Azam from the GMB union said: "There is a proven track record of top people paying themselves higher incomes as soon as they get control of the money.
"It happens in the privatised water, gas and electricity sectors as it happens in the banks. "Salary levels of GPs are likely to escalate towards the stratospheric levels we see in the USA."
She said the bill "introduces a potential conflict of interest with GP Fund Holders having the possibility of putting their own Funds before the needs of patients".
"It opens the door to GPs helping themselves to even larger amounts of public money to go into their own pockets and there is little in the Bill to stop them," she claimed.
But the British Medical Association dismissed the claims. A spokesman said: "That is not our interpretation of the Bill. There is an existing contract in place that determines GPs' pay, and we do not expect that to change because of the Bill."
She added that any funding for commissioning services would be "completely separate from practice income".

The GPs won't be able to increase their allocations. All they can do is control where the money goes. It will be heavily scrutinised - I sincerely believe it's a trap to destroy General Practice and lead to privisation of the NHS.

Who the [LIFTED] cares. They work hard and spent a long time training. I'd rather they had plenty of money than some jug eared granny [LIFTED] leather kicker.