In a controversial finding that is certain to infuriate the Lawrences, Sir William Macpherson of Cluny, chairman of the public inquiry, plans to question Mr Mansfield's professional judgement in allowing the family to pursue the case.
The family's solicitor, Imran Khan, who has a reputation for championing high-profile anti-Establishment cases, faces similar criticism over the private prosecution, which led to three of the five prime suspects being acquitted of Stephen's murder.
Sir William's report is in its final stages and is expected to be delivered to the Home Secretary, Jack Straw, next week.
The Independent understands from legal sources that Mr Mansfield, a QC, and Mr Khan received letters before Christmas warning them that they would be censured in the report, which will be published in mid-February. They are understood to have made a spirited defence of their actions in written responses to Sir William and his advisers.
Neville and Doreen Lawrence, Stephen's parents, embarked on the private prosecution - such a rare course of action that it was only the fourth case in 130 years - after the Crown Prosecution Service (CPS) discontinued its case against the five suspects: Jamie Acourt, Neil Acourt, David Norris, Gary Dobson and Luke Knight.
Three of them, Knight, Dobson and Neil Acourt, stood trial at the Old Bailey in 1996. But Mr Mansfield, who led the prosecution, was compelled to abandon the case after the judge, Mr Justice Curtis, ruled that crucial identification evidence by the main witness, Duwayne Brooks, was inadmissible. Mr Brooks, Stephen's schoolfriend, was diagnosed with post-traumatic stress after the murder, and Mr Justice Curtis said his evidence was "contaminated" and unreliable. On the judge's direction, the three defendants were formally acquitted and, under English law, can never again be tried for the same offence - even if compelling new evidence were to emerge.
The publicity that accompanied the private prosecution - not to mention the subsequent inquest and public inquiry - would almost certainly enable the other two men to argue that they could not now receive a fair trial.
Stephen, 18, was stabbed to death by a white gang in an unprovoked racist attack as he and Mr Brooks were waiting for a bus in Eltham, south-east London, in April 1993. Sir William's report will criticise 23 Metropolitan Police officers over the bungled murder investigation.
Mr Khan declined to comment last night, and Mr Mansfield failed to return telephone calls. Both men are believed be angry at the prospect of being criticised by Sir William, a retired High Court judge.
The Lawrences are bound to be upset by the attack on their lawyers, who have spearheaded their six-year campaign for justice and become close family friends in the process.
Mr Khan became the family's representative in the immediate aftermath of the murder, while Mr Mansfield's involvement dates from late 1993. They are both thought to have waived their fees during certain periods.
The wisdom of launching the private prosecution has been questioned repeatedly over the years, since the evidence of Mr Brooks - who witnessed the murder - was always known to be problematic.
Stephen's parents maintain they had no choice but to take action themselves after the CPS discontinued its case against the suspects in July 1993.
After Knight, Dobson and Neil Acourt were committed for trial in 1995, Mrs Lawrence described the CPS's decision to drop the charges as "an act as hurtful and as painful in its effect as the news that Stephen had been killed".
The Lawrences still hold the view, publicly at least, that the three men would be behind bars if the matter had been left to the Old Bailey jury.
Howard Youngerwood, the senior CPS lawyer who took the decision to discontinue the original prosecution, launched a strong attack on Mr Mansfield and Mr Khan at the public inquiry.
He said that by insisting on going ahead with the private prosecution they had wrecked all chance of bringing Stephen's killers to justice. There had been no hope of a conviction on the evidence available, he said.
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