Iain Anderson said the situation for the APPC’s 80 members was "far from ideal" because many would have to retro-fit their operations towards guidelines from the Office of the Registrar of Consultant Lobbyists, which were still unclear.
In particular, the APPC is concerned about creating a level playing field with law firms as well as management consultants that carry out lobbying work.
It also questioned how long registered lobbyists would have to pay the annual registration fee of around £750 if they did not carry out any work in a given financial year, a so-called nil return.
Anderson called on the ORCL to "look again" at the issue of using Twitter to contact ministers.
He said: "We believe tweeting is part of an open debate that anyone can take part in and that the guidance on tweeting is wrong. There are so many questions still to be answered by the ORCL, with only 25 days to go until the first wave of registrations are completed. We think it has left a lot of issues until five-to-midnight to resolve and that is far from ideal because people have to run their businesses on the basis of guidance that is not complete."
Anderson said the APPC would continue to provide rolling guidance for its members to help them navigate the regulations emerging from the ORCL.
The ORCL was contacted but was unavailable for comment at the time of going to press.