The letter from the Royal College of General Practitioners (RCGP) said that many practices are struggling to recruit and retain family doctors.
It said urgent funding is needed to stop practices closing.
In December, hundreds of doctors signed undated resignation letters because of funding concerns and workforce strain.
A rescue package, which would have increased training places, was then agreed with the health minister.
However in the open letter the RCGP said that the current political deadlock at Stormont means plans to improve GP care have had to be put on hold.
The letter points to the "grave challenges and pressures facing the health and social care service in Northern Ireland".
"Patient waiting lists continue to grow, patient care is at risk and GP morale is plummeting," it said.
On Monday the chairman of the NI General Practitioners' Committee said Northern Ireland's GPs would likely vote to leave the NHS if a snap election is called.
Dr Tom Black said that recent weeks have "clearly shown that general practice here is on the edge of a full-blown crisis,".
"It is catastrophic," he added.
The letter comes after Bannview Medical Practice in Portadown faced closure after its last remaining doctor resigned. It has 5,200 patients on its books.
A new contractor, who is based in Belfast, had agreed to take over the practice, but that GP has now withdrawn from the contract.
GPs in Northern Ireland have written an open letter to Stormont politicians to warn them that services are on the verge of collapse.