The Chilcot inquiry is incapable of addressing the key issue of whether the invasion of Iraq was legal, senior judicial figures have said, adding to the controversy surrounding the inquiry's legitimacy.

The inquiry into one of the most contentious political decisions of modern times begins hearing evidence tomorrow, and its chairman, Sir John Chilcot, has insisted that the legality of the invasion in 2003 will be one of the key issues it addresses.

But one senior judge told the Guardian that analysing the war's legality was beyond the panel's competence.It does not include a single judge or lawyer.

"The truth of the matter is, if the inquiry was going to express a view with any kind of authority on the question of legality, it would need a legal member and quite a senior one," the judge said. "Looking at the membership ? it seems to me that legality just wasn't going to be a question they would be asked to review."

Another senior legal figure said: "The panel clearly lacks the expertise to address the question of legality. The members are not experienced at cross-examination ? it is simply not their skill set."

The criticisms come after the chairman of the inquiry has been repeatedly forced to defend its approach amid claims that the process is a "whitewash".

There have been repeated calls from influential legal and judicial figures for an investigation into whether the invasion of Iraq was illegal, including the former senior law lord Lord Bingham, who last year reiterated that it was "a serious violation of international law".

Sir John Chilcot, a former senior civil servant who was criticised for adopting a "light touch" in some aspects of his questioning during the Hutton inquiry and whom critics regard as having strong links with the establishment, appeared to acknowledge demands for an investigation of legality recently by confirming that Tony Blair, and possibly the former attorney general Lord Goldsmith, would be called to give evidence.

But scrutiny of the panel's lack of experience on law and cross-examination techniques raises questions about the willingness of the government, which established the inquiry, to look seriously at whether the government acted illegally.

"Some of the debates around the legality of the war are quite sophisticated ? it is not all clear-cut," the senior legal figure said. "It's going to be very difficult to deal with someone like Blair without a panel experienced in cross-examination.".

"Looking into the legality of the war is the last thing the government wants," said the judge. "And actually, it's the last thing the opposition wants either because they voted for the war. There simply is not the political pressure to explore the question of legality ? they have not asked because they don't want the answer."

Last month it emerged that Dame Rosalyn Higgins, former judge and president of the international court of justice ? the world's highest court ? was to be a legal adviser to the panel. But Higgins, who is highly regarded in legal circles but also said to be an "establishment figure", will not sit on the panel or cross-examine.

"Lawyers are trained to weigh up evidence and will know and say when they see a decision-making process that appears to be out of the ordinary," said the British international law expert Professor Philippe Sands QC. "The fact that the members of the inquiry do not include a lawyer is very, very telling".

Experts also criticised Chilcot's statement todaythat the inquiry was "not definitive in the sense of a court verdict". "That's an avoidance strategy" to "keep control of the agenda", one said.