Martin McGuinness, Northern Ireland's education minister, is to tell the official inquiry into the events of Bloody Sunday that at the time of the 1972 shootings he was second in command of the provisional IRA's Derry unit.

Reports of McGuinness' admission to the inquiry, which is chaired by Lord Saville of Newdigate, have led the Democratic Unionists to call for his arrest for IRA membership.

The statement will be the first time that a leading Sinn Fein figure has admitted to an IRA role and will give credence to Unionist claims that Sinn Fein and the IRA are one organisation. The admission will also involve breaking the oath of secrecy that IRA members swear.

Revealing details of his role within the IRA for the first time, McGuinness is expected to say that the IRA were asked to support a peaceful march and accepted to the proposal. At a meeting the day before the march with the IRA's officer commanding it was agreed that members of the IRA would not engage with the security forces.

It is believed McGuinness will also say that two IRA units were armed and told to stay in the Creggan and Brandywell areas while all other members were given the choice of attending the march or spending the day with their families.

The civil rights demonstration on Sunday January 30, 1972, resulted in the shooting of 13 civilians, with another man dying later from his injuries, by British paratroopers.

Although the inquiry has received evidence from a security service agent that McGuinness fired the first shot, he is likely to dismiss this as a lie.

Ian Paisley Junior, DUP justice spokesman, has called for McGuinness to be arrested and charged with membership of the IRA if he admits his role.

"The indications reported on the media that he will admit that he was an IRA commander ought to be used to ensure that, because he is education minister in the executive will not prevent justice being done," said Paisley.