London-based human rights lawyer Jennifer Robinson said on Thursday Heathrow airport authorities had informed her she was on an "inhibited travel list" and needed special permission to return to Australia.
She said on Twitter security guards had told her "you must have done something controversial" because we have to phone the embassy. Australian lawyers and human rights organisations have reacted angrily to the incident.
Ms Roxon, a former lawyer, is due to speak at Commonwealth Regional Law Conference in Sydney on Friday and is expected to face a grilling on the issue. Ms Robinson was travelling to Sydney to also address the lawyers conference.
The departments of Foreign Affairs and Immigration have denied any knowledge of travel restrictions on Ms Robinson and said UK border authorities and the airline might be able to provide insight into the claims.
Commonwealth Law Association former president Ron Heinrich told AAP if the claims were true the treatment of Ms Robinson was "unprecedented."
He said the incident raised "profound issues concerning the independence of lawyers and their clients."
"I would anticipate there will be lots of people in the audience who will ask (Ms Roxon) about it," Mr Heinrich said.
Australian Greens senator Scott Ludlam called on Foreign Minister Bob Carr to explain the purpose of the list, who compiles it and whether Ms Robinson's legal representation for Mr Assange was connected to her being on a list.
Australian Lawyers Alliance national president Greg Barns said he was gravely concerned by Ms Robinson's treatment.
"Simply because a lawyer is representing a client that government does not like, the lawyer is then subjected to security scrutiny," he said.
"Lawyers must be able to act for their clients without fearing that they will be harassed by government agencies, either in their own country or overseas."