Lord Saville of Newdigate is due tomorrow to publish his findings on the killings of 14 nationalist protesters by British soldiers in Derry in January 1972. His inquiry, initially expected to take less than two years, has taken more than 12 years and has become the longest-running in British history.
It cost the public more than £191m with more than £100m spent on lawyers involved in legal hearings.
It is believed to conclude that some of the protesters were killed unlawfully. Prime Minister David Cameron is preparing to apologise, if necessary, government sources said.
Northern Ireland's Director of Public Prosecutions will consider whether to bring charges against any of the soldiers after the report has been published.
Lawyers for the victims' families say they will press for all those who opened fire to be charged on a "joint enterprise" basis. This prosecution is used when it is unclear which member of a gang killed someone.
Bloody Sunday Footage
Greg McCartney, who represents one family, said: "Anyone who fired a shot could come within it."
Meanwhile, Justice Secretary Ken Clarke yesterday branded the Bloody Sunday inquiry under Lord Saville as disastrously expensive and time-consuming.
Mr Clarke said the Saville inquiry had grown "ludicrously out of hand" and must not be repeated.
Mr Clarke said the inquiry -- set up by Tony Blair in 1998 -- had been a "disaster in terms of time and expense".
"I'm anxiously considering how we can stop such inquiries getting ludicrously out of hand, in terms of cost and length," he said.
Ex-army figures yesterday argued it would be unfair to prosecute British soldiers while IRA terrorists remained free. Martin McGuinness, the IRA No 2 in on Bloody Sunday is now Northern Ireland's deputy first minister.
Field Marshal Lord Bramall, the British army commander during the Falklands War, said: "When you think of all the pardons that have been dished out to the IRA, I think it would be terribly wrong all these years later if undue punishment was dished out on British soldiers."
A serving Parachute Regiment officer said soldiers were "deeply unhappy" about the inquiry and questioned "whether prosecutions would be in the public interest".
"It still rankles that not one of the IRA perpetrators who still remain at large will face public inquiry," he said.