A couple who fostered the Parsons Green bomber are suing Surrey County Council for negligence, after not being told he had been "trained to kill" by the Islamic State group.
Ron and Penny Jones fostered Ahmed Hassan, whose homemade bomb injured 51 people on a London Tube train in 2017.
The couple, who have looked after nearly 270 children, have not been allowed to foster any more children and say their lives are "empty".
The council is defending the claim.
Hassan, an Iraqi asylum seeker, was jailed for life aged 18 last year for the Parsons Green attack, which he said he carried out after contacting Islamic State militants.
He had made the bomb at the kitchen table in his foster home, while his foster parents Mr and Mrs Jones were away on holiday.
Couple not told of IS training claims
Mr and Mrs Jones have been foster carers for 40 years and received MBEs from the Queen in 2010.
Surrey County Council - which placed Hassan with Mr and Mrs Jones - did not tell the couple that in a January 2016 immigration interview, Hassan had told officials he had been "trained to kill" by the Islamic State group.
That information was not mentioned in the risk assessment prepared prior to the foster placement.
The couple only found out during Hassan's trial and were horrified.
Mr and Mrs Jones had raised concerns about Hassan with the council. They reported that he had been taking calls in the middle of the night and had multiple phones.
Last June, a review into the handling of Hassan revealed a series of errors by Surrey County Council and the police.
Following Hassan's trial, Surrey County Council stopped the couple from fostering. Now the Joneses are taking unprecedented legal action against it.
'Didn't tell us the truth'
Penny Jones told the BBC: "How was it our fault they put him here, they didn't tell us the truth, they should have been honest with us to start with."
"We've lost everything, we've lost our income, we've lost our will to get up in the morning, because our life has revolved around children for over 40 years. Our life is empty."
She said she hopes that the legal action - which is being crowdfunded - will ensure that other foster carers do not suffer as they have.
"I don't want anyone else to go through what we've gone through," said Mrs Jones. "We need social services to be open and honest, and to tell you the truth.
"If you've got a trained killer, you need to know you've got that trained killer."
What was the Parsons Green bombing?
On 15 September 2017, horror descended on a London Tube train.
A partially detonated bomb sent a fireball down a carriage, burning morning commuters who stampeded to escape.
A total of 23 people suffered burn injuries and 28 received crush injuries as they fled from the train and station. The bomber was 18-year-old Ahmed Hassan.
Prosecutors at the sentencing hearing said there would have been "serious harm, if not fatality" if the bomb - packed with 2kg of screwdrivers, knives, nuts and bolts - had fully detonated. The judge branded Hassan "dangerous and devious".
Mr and Mrs Jones have both suffered health problems as a result of their time with Hassan, the bombing, and the criminal trial.
Mr Jones has been diagnosed with angina and suffers night terrors recalling the bomb made on the kitchen table, and what might have happened if Hassan had turned on them.
"I still get dreams about what might have happened, but to think he had enough to blow this place to pieces..." said Mr Jones. "If we had known he was a trained killer he wouldn't have got over the doorstep."
In legal papers seen exclusively by the BBC, Ron and Penny Jones are claiming negligence and breach of their human rights against Surrey County Council.
The papers state: "In breach of this duty of care, your client failed to disclose that Ahmed Hassan had been and/or had said that he had been 'trained to kill' by ISIS and falsely represented that Ahmed Hassan was simply a very vulnerable young person who had been deeply traumatised by his experience in Iraq and was suffering from mental health problems as a consequence of those experiences."
The papers go on to say that, but for the breaches of duty, "Mr and Mrs Jones would not have agreed to become Ahmed Hassan's foster carers."
The case has garnered support from other foster carers, who on Wednesday staged a protest outside Surrey County Council's County Hall in Kingston Upon Thames, demanding justice for the couple.
A petition with 2,500 signatures was handed in to the director of children's services, Dave Hill. When the demonstrators entered the building, Penny Jones was asked to leave.
A wider problem for foster carers
Sarah Anderson, chair of the Foster Care Workers Union, believes the treatment of the Joneses by Surrey is indicative of a wider problem.
"Penny and Ron were literally thrown on the scrap heap due to Surrey's failings," she said.
"We are kind of disposable, we live in the most precarious of circumstances where we are not classed as workers, though we actually are. We have no rights whatsoever, and this just shows how disposable we are."
In a statement, a spokesman for Surrey County Council said: "We are defending this claim however we acknowledge this has been a very difficult time for Mr and Mrs Jones and their family.
"We place a high value on openness with all our foster carers, share information about any risks with them from the outset and continue to keep them informed.
"This was our approach with Mr and Mrs Jones."
Jocelyn Cockburn, a specialist civil liberties solicitor from Hodge Jones & Allen, who represents Penny and Ron, said: "Surrey Council owed Penny and Ron a duty to disclose key information about the danger Ahmed posed, and to ensure any information given was accurate, before asking them to invite him into their home.
"Instead he was presented merely as a troubled young person. This is a clear failing by the Council and gives rise to claims in negligence and under the Human Rights Act".