Mr Clarke marked himself out as a hostage to fortune recently when he pledged during a BBC TV Question Time that he would resign if Lord Justice Scott criticises him in his final report.
In what could turn out to be the most difficult phase of the inquiry for the Government, Mr Clarke's evidence will be followed by other ministers who signed certificates: Malcolm Rifkind, Secretary of State for Defence and like Mr Clarke a former lawyer, Peter Lilley, a former trade minister, now Secretary of State for Social Security, Tristan Garel- Jones, former Foreign Office minister, and Michael Heseltine, who has also undertaken to resign if criticised.
The final witness, due to be examined on 17 March, will be Sir Nicholas Lyell QC, the Attorney General. He is expected to mount a strong defence of his role, repeating the view he spelt out to the House of Commons in November 1992 that ministers had a duty to assert the immunity, and that the certificates were not 'gagging' orders because Matrix Churchill defendants had the ultimate safeguard of the judge deciding where the balance of public interests lay.
At the very least, Lord Justice Scott is likely to make strong recommendations to improve the procedures when the immunity is invoked. He has already voiced strong criticism of the ready acceptance by government lawyers that suppression of whole classes of official information can be defended on the grounds of public interest.
The Chancellor of the Exchequer has no doubt he is a contender for the job of Prime Minister - but only when John Major retires. The admission came yesterday when he appeared as a guest on BBC Radio 4's Desert Island Discs.