The Bloody Sunday inquiry into the massacre of 13 unarmed civilians at a civil rights march by British soldiers of the Parachute regiment in Derry in 1974 will conclude that a number of the killings were unlawful. A 14th person died later.

That opens up the prospect of some of the soldiers being tried for manslaughter if not murder.

One soldier in particular, who is said to have been responsible for at least four killings, may well find himself in the dock. It will be up to the Public Prosecution Service to decide.

"Soldier F" was identified by the families as the worst offender on the day, killing people at random. He was warned during the inquiry that his evidence amounted to perjury.

The sensational findings will be fully released on Tuesday next in the House of Commons. Prime Minister David Cameron will make a statement before it is released.

The 12 year inquiry was headed by Lord Peter Saville, the longest public inquiry in British history.

The findings will cause consternation, especially on the unionist side.

Lord David Trimble, the former leader of the Ulster Unionist party told the Guardian newspaper that when Tony Blair originally agreed to the inquiry in 1998, he told him any conclusion that departed "one millimetre" from the earlier 1972 Widgery report into the killings would lead to "soldiers in the dock".

"I just reminded him that the Widgery report of 1972 concluded that the troops' behaviour, to quote from the report, 'bordered on the reckless'.

"Then I told the prime minister that if you moved from one millimetre from the that conclusion you were into the area of manslaughter, if not murder," he said.

"I pointed out to Blair that we would see soldiers in the dock. I told him that at the time of the talks leading to the Belfast agreement," Trimble said.

One unionist MP told the newspaper the conclusion of unlawful killings is a "hand-grenade with the pin pulled out."

Up to 10,000 people are expected to march in Derry on the day the inquiry is released. They will take the same route as the march on Bloody Sunday.

Tony Blair Blair and his then Northern Ireland secretary, Mo Mowlam, announced the Saville inquiry on 30 January 1998 – on the 26th anniversary of the shootings and said there was "compelling new evidence".