LMCs say PCTs have paid only half of the 14 per cent employer's superannuation contribution for staff employed from growth funding received by PMS practices in 2004/5 and 2005/6.
However, under the standard Lockharts contract used by most PMS practices, PCTs must fund employer's superannuation contributions in full.
The outstanding 7 per cent has cost GPs around £2,000 each.
Across Avon the total unpaid amount over two years comes to around £840,000.
About 40 per cent of practices in England are PMS, and it is believed up to £28 million is owed in total.
Some PCTs say they did not receive superannuation funding for staff employed from growth money. But specialist healthcare solicitors Lockharts investigated PCT accounts and told Avon LMCs that 'PCTs were properly funded to allow them to pay' the outstanding 7 per cent.
Avon LMCs gave five PCTs in the area until 31 January to pay up, but LMC chief executive Steve Mercer said: 'I am not aware that any PCTs have paid this money.
'PCTs have no excuse. We have formally asked practices to come forward as test cases.'