The Home Secretary must order a public inquiry into undercover police who spy on political campaigners or the family of murdered black teenager Stephen Lawrence will sue, human rights campaigners were told.
In a statement to help launch the Campaign Opposing Police Surveillance in central London, Baroness Lawrence said that only "the most authoritative, public, transparent and legally-robust framework possible" to investigate police actions will be enough to satisfy both her own and the public's concerns on this "important" issue.
Police chiefs have conceded that undercover officers were deployed to spy on supporters of her family's efforts to force Scotland Yard to investigate her son's racist murder properly.
Her lawyer Imran Khan said: "She ( Home Secretary Theresa May) should order an inquiry and we are going to challenge it legally if she doesn't."
Claims the police tried to smear the Lawrence family in the aftermath of Stephen's murder are being investigated by two existing inquiries.
The allegations are being examined by an internal police investigation, known as Operation Herne, run by the Derbyshire chief constable, Mick Creedon.
Campaigners say they have "no faith" in Operation Herne or any of the other internal official inquiries to uncover the truth of the undercover infiltration that started in 1968.
Barrister Mark Ellison QC, who successfully prosecuted Gary Dobson and David Norris for Stephen's murder in 2012 - is examining police corruption during the original investigation in to the killing.
It comes after undercover officer Peter Francis said he was instructed in 1993 to find information that could discredit the Lawrence family. He claims that he posed as an anti-racism campaigner in a hunt for "disinformation" to use against those criticising the police.
In her statement, read out by Mr Khan, Baroness Lawrence stressed: "Only a judge-led public inquiry can perform that task. A police review and a review by an experienced prosecutor can, no matter how carefully or thoroughly conducted, simply will not have the necessary level of credibility and authority to draw a line under these important issues."
Baroness Lawrence said that, along with members of the public, her fear is that a police or barrister-led review will only seem as though they are "shielding the police from public scrutiny".
She believes "very strongly" that an inquiry should cover a wide range of people who say they are victims of this type of surveillance, Mr Khan said.
The new campaign brings together people who say they have been targeted by this undercover work. They include women who say they were duped into forming long-lasting relationships with undercover police such as London Greenpeace activist Helen Steel. She was one of the pair in the notorious McLibel case sued by burger giant McDonalds over a leaflet.
Stephen, 18, a would-be architect, was stabbed to death by a group of up to six white youths, in an unprovoked racist attack as he waited at a bus stop in Eltham, south east London, with a friend on April 22 1993.
It took more than 18 years to bring two of Stephen's killers to justice. An inquiry following the murder led the Metropolitan Police to be accused of institutional racism and found failings in how the force had investigated the crime.
Mr Khan said he feared that if Mr Francis was called to give evidence behind closed doors that nobody would ever hear about it.
He noted: " One of the things that was instrumental in the public inquiry into Stephen Lawrence's murder was the ability to hold all of that out in the open and for officers to be held to account in ways that had never happened before.
"If we allow the Home Secretary to say there is going to be no inquiry into all of this, we are going to be left in a situation where he police can act with impunity, where Creedon can say 'I am going to go against you with the Official Secrets Act' and people will never know and be able to get to the bottom of what happened.
"There have been 20 or 30 years of undercover policing in this country which has only just come out. It is shocking."
Ms Steel, who is among a group of women who have launched a legal action against the Metropolitan Police, says she was targeted in 1990 by undercover officer John Dines, who used the alias John Barker.
She believed they were "soul mates" and had talked about starting a family - but he was actually married and had adopted the name of a dead child.
Now describing the relationship as "a deliberate process of emotional manipulation," she told the campaigners: "He was seeking to draw me closer to him so that he could spy on me and my friends and seek to undermine the political movements I was involved with and, ultimately, seek to prevent change."