One practice, which has not been named, stands to lose £219,000 from its annual income.
Northumberland Care Trust has made offers to practices after several months of negotiating changes to the PMS contract. The trust has been visiting individual practices to clarify the terms, which will come into effect from April.
The trust is currently £14 million in debt and hopes to break even in April 2008.
PMS GPs in the area face large cuts to their baselines as the trust struggles to recover £2 million from PMS contracts.
Northumberland LMC secretary Dr Jane Lothian said the reports from practices were that there was little flexibility so far.
She added that there was a worry that the cuts could affect the viability of smaller practices and would have a negative effect on patient care.
'We have a long history of co-operation with the PCT but this will make it harder to deliver the standards they require,' she said.
A Northumberland Care Trust spokeswoman said that it had been scrutinising all aspects of spending as part of its financial recovery plan. This included reviewing the cost of PMS agreements with GPs, she said.
The trust said it would also cap any reductions in budget and offer £10,000 to some small practices to 'offset core costs'.
The renegotiated contracts will affect 40 PMS practices covering 288,000 patients - 90 per cent of the local population.
GPC chairman Dr Laurence Buckman said that he hoped the practices would unite and collectively reject the offer.
'They should stand up and say they will not be pushed around. It is just bullying.'
PMS practices in Suffolk are in a similar position.