Attorney-General George Brandis is facing an embarrassing defeat as Labor and the crossbench unite to block a controversial move by him to restrict access by his colleagues to the government's chief legal adviser.

Senator Brandis is locked in an extraordinary brawl with Solicitor-General Justin Gleeson, SC, over a binding direction he issued requiring all ministers, including the Prime Minister, to obtain his written permission before seeking advice from Mr Gleeson.

Attorney-General George Brandis has faced calls to resign after Labor accused him of misleading Parliament. Credit:Fairfax Media

Legal experts have expressed concern the move could mean Mr Gleeson, who provides apolitical advice on major issues and appears in high-profile cases on behalf of the Commonwealth, is "frozen out" of advising the government.

Fairfax Media has revealed Senator Brandis sought the opinion of Howard-era solicitor-general David Bennett, QC, instead of relying on Mr Gleeson's advice on the same-sex marriage plebiscite.