They have calculated that an extra 10,000 GPs are needed in the health service in England and Wales - an increase of almost a third on existing numbers.
This is five times the number that the government has promised.
The warning comes from the British Medical Association (BMA) and the Royal College of General Practitioners (RCGP).
In a joint report, they say that current targets for increasing the number of family doctors are "nowhere near" the levels needed to implement the reforms laid out in the government's NHS Plan.
The report says an extra 10,330 full-time GPs are needed in the medium to long term.
In the NHS Plan published in July, the Government promised an extra 2,000 GPs by 2005.
Maximum wait
But the BMA-RCGP report said that the targets of the plan meant thousands more staff would be needed.
Targets set out in the NHS Plan include:
"Now the NHS Plan raises expectations and identifies new work for general practice, but there are nowhere near enough GPs to deliver what is expected."
Professor Mike Pringle, chairman of the RCGP, said: "Research suggests that less than a third of junior doctors are interested in becoming family doctors, but around half of all UK medical graduates will need to enter general practice if the appropriate balance between GPs and specialists is to be maintained."
The number of doctors newly qualified to work as GPs in the NHS is at a 15-year low.
The recruitment problem is compounded by the imminent retirement of large numbers of GPs, many from overseas, who work in the most deprived areas of the UK.
There is also a general trend towards earlier retirement throughout the GP workforce in common with trends throughout society.
The BMA and RCGP have called for a recruitment drive.
It also wants measures to reduce the pressure on GPs, such as smaller patient lists and a greater emphasis on delegating clinical and administrative work to other members of the primary health care team.