The comments, by French Socialist member Sylvie Guillaume, follow a report this week by transparency group Alliance for Lobbying Transparency and Ethics Regulation.
It found that too many law firms in Brussels are absent from the EU's current lobby transparency register "while clearly running EU lobbying services."
ALTER-EU said the "boycott" of the voluntary EU lobby register is further evidence to back its demand for a legally-binding register.
But Guillaume, a Vice-President of the European Parliament, told this website, "The issue of law firms is not a simple one and I'm not sure the upcoming negotiations on the revision of the transparency register will be able to tackle it completely.
"Why? For a simple reason: a change is needed in the European countries. Indeed we still have various situations from one country to another.
"For instance, Spanish and Portuguese constitutions defend the absolute right to confidentiality for all persons seeking legal counsel, while the French bar has changed its rules to allow law firms to lift the client confidentiality clause, when the client agrees."
An upgrade of the EU lobbyist register is planned after the European Commission launched a public consultation on switching from the current voluntary register to a mandatory one for anyone interested in speaking to EU institutions to influence their work.
Guillaume took part in a debate in Parliament on the issue earlier this month with experts and European Commission First Vice-President Frans Timmermans.
The European transparency register was set up in 2011 after the Commission and Parliament agreed to have a joint register.
Registration is currently voluntary, but the number of registrants has increased steadily in recent years. There are now more than 9000 organisations on the list, with about 1000 new entries per year.
Guillaume says the transparency of EU decision-making is an important issue for the Parliament.
She said, "On many occasions, Parliament has called for a mandatory register of lobbyists.
"We are also happy that the Commission now shares Parliament's position and we are determined to go ahead together and ensure that the Council plays its part as well."
The Commission launched a public consultation on a mandatory system in March and stakeholders were asked to share their views by 1 June. The Commission's move followed up on the pledge made by its President Jean-Claude Juncker at the start of his term to pursue more transparency and to propose a mandatory lobby register.