The Pharmaceutical Services Negotiating Committee told the BBC the Department of Health has drawn up proposals to cut funding by 12% from December.
It said the cuts were "madness" and would damage the NHS and social care.
The Department of Health said no final decision had been made. An announcement is expected shortly.
Sue Sharpe, chief executive of the PSNC, told BBC Radio 4's Today programme the changes would throw the health service into "chaos", as more people would be forced to turn to GPs instead of pharmacists.
In a letter to the Department for Health seen by the BBC, she wrote: "The proposals were and remain, founded on ignorance of the value of pharmacies to local communities, to the NHS, and to social care, and will do great damage to all three. We cannot accept them."
The PSNC - which negotiates with the Department of Health and NHS England on behalf of pharmacies - said it had been told pharmacies would receive £113m less than expected from December 2016 to March 2017 and £208m less the following year.
The cuts amount to 12% in the coming months and 7% for the next financial year, compared with current spending.
Earlier this year, the then health minister Alistair Burt suggested between 1,000 and 3,000 pharmacies could be closed after spending cuts, although the Department of Health now says it does not recognise the figure.
Officials have been negotiating cuts to the money pharmacies receive from government since last December.
Plans for a £170m cut this year were delayed after 2m people signed a petition opposing the change.
Pharmacies get around 90% of their income from the money government pays for dispensing prescriptions. It costs the taxpayer £2.8bn a year.
A Department for Health spokesman said it was investing £112m to put 1,500 pharmacists in GP's surgeries.
He said: "We have worked collaboratively with the PSNC and have listened to their suggestions and counter proposals over the course of many months.
"Ministers are considering a proposed package for the sector and no final decision has been taken, but we are committed to offering more help to those pharmacies people most depend on compared to others."