Patients turned down for payments are likely to vent their frustration and anger on their GPs rather than the politicians responsible, they believe.
Charities will also ask the Government to drop rules introduced in April, under which claimants who do voluntary work are told that if they are fit enough to volunteer they are fit for work.
Groups of volunteers have already resigned, according to Richard Gutch, chief executive of Arthritis Care. 'Volunteers will leave in droves unless the Government drops this regulation. Without volunteers most charities could not function,' he said.
This could undermine a key element of the Government's policy of moving services now peformed by the state to the private and voluntary sector. For instance the community care policy introduced in April relies on charities to help to support old and sick people.
Ian Bogle, chairman of the general medical services committee of the British Medical Association, which represents 30,000 GPs, is pressing Virginia Bottomley, Secretary of State for Health, to use her influence in Cabinet to scotch proposals for more stringent medical tests, disclosed last week.
A sharp rise in violent attacks by patients on GPs was revealed this month in a BMA survey of nearly 2,000 doctors.
One in 12 London GPs has been assaulted on duty in the past two years, and two in five threatened by patients or their relatives. Gerry Flack, a GP from Ashford, Kent, survived being shot three times while on his rounds three weeks ago.
Dr Bogle said that stiffer criteria for medical certificates - which enable the long-term sick to go on receiving invalidity benefit - would increase the chance of confrontation.
He described as 'absolutely scandalous' the reported views of some ministers that GPs were partly to blame for the pounds 6bn invalidity benefit bill.
'I have never known a colleague give a certificate dishonestly, but sometimes it is difficult to decide whether someone is fit to work. It is not the job of GPs to police the benefit system.'
At present doctors can sign someone off sick for a month before the patient has to return for examination. After six months on sickness benefit, averaging pounds 45 a week, a patient can apply for invalidity benefit of pounds 79 a week.
The National Council for Voluntary Organisation, which represents almost 700 charities is to carry out a survey of members and volunteer bureaux to establish how widespread the threat to volunteers' benefits could be.