The Campaign for Freedom of Information said civil servants practised in the art of preserving secrecy by evading MPs' questions would apply identical techniques on the public.
Maurice Frankel, the campaign's director, said the refusal in last month's White Paper to allow direct access to documents marked it out as a 'vastly inferior product'.
In most cases, disclosure would be limited under a voluntary code of practice to the release of selected and 'laundered' information - not correspondence, documents or reports. Even this could not be imposed because the Parliamentary Ombudsman would be restricted to recommending disclosure, not compelling it.
Mr Frankel said: 'Access to records is a guarantee of the truth. People can see the real details: the unedited facts, assumptions, contradictions, oversights or mistakes.'
The campaign, co-chaired by the Liberal Democrat, Conservative and Labour MPs Archy Kirkwood, Richard Shepherd and Chris Smith, said access to information without documents, the basis of the parliamentary questions system, would bring nothing new: 'Civil servants who have spent a lifetime perfecting techniques of evading MPs' questions will now be free to practise on the general public.'
The code would confine disclosure to 'factual' information, leaving departments to decide whether to volunteer expert advice or analysis. They would also have the right to keep secret information considered to be 'unreliable' or 'misleading'.
Mr Frankel said: 'When figures unexpectedly suggest that a government policy is having the opposite effect to that intended, the Government probably does regard them as unreliable - so they won't be available.'
Secrets, the campaign's newspaper, contains a response to the White Paper; 88 Old Street, London EC1V 9AR; pounds 12.50 a year.