Former British Prime Minister Edward Heath is due to give evidence to the Bloody Sunday Inquiry today.
The former Conservative leader, 86, is only the second prime minister to appear before a British government-appointed public inquiry.
Baroness Thatcher appeared before the inquiry into the arms-for-Iraq affair in December 1993.
The Bloody Sunday Inquiry, which opened five years ago, is investigating the circumstances surrounding the killing of 13 men by British paratroopers on a Derry civil rights march on January 30, 1972.
The bloodshed on the Bogside, which has since become known as Bloody Sunday, was a grim milestone in the Northern Ireland troubles and helped underpin 30 years of violence before a peace deal could be struck.
Bereaved relatives will watch as Heath gives evidence over three weeks at Westminster Central Hall in central London.
Heath will face questioning on whether there was a cover-up of the shootings.
He will also be asked about claims that Lord Hailsham, then the Lord Chancellor, told a ministerial meeting that anyone who obstructed the army could be shot as enemies of the Queen.
The killings were carried out by the 1st Battalion of the Parachute Regiment, who said they only fired at gunmen and nail bombers. They had been ordered into the Bogside to arrest rioters.
Campaigners believe they fired indiscriminately and without justification.
The original inquiry, which was ordered by Heath immediately after the shootings, largely exonerated the soldiers saying they fired in self-defence.
Survivors and the bereaved have dismissed it as a whitewash.
Heath is among 350 political and military witnesses who are testifying in London for safety reasons.