Their alleged captives - a 30-year-old Briton, a 69-year-old Malaysian and a 57-year-old Irish woman - were released by police and staff from the London-based Freedom Charity several weeks ago.
Such is the women's fragile state that officers have had to spend days painstakingly piecing together details of their alleged ordeal before moving in to make arrests.
The call that ended their apparent captivity came after one of the two older women had watched a documentary about forced marriage which focused on the Freedom Charity; she used a freephone number at the end of the programme to contact astonished welfare workers.
Detective Inspector Kevin Hyland, who is leading the investigation, said the women's alleged servitude was three times longer than anything seen before by police.
He said: "We have launched an extensive investigation to establish the facts surrounding these very serious allegations. All three women were deeply affected and traumatised. Their lives were greatly controlled, and for much of [the time] they would have been kept on the premises.
"We have had some previous cases...where we know people have been held against their will for up to 10 years. We have never seen anything of this magnitude."
The Yard said many details of the women's lives over the past three decades, including just how they came to be held in the house in the first place, remained unclear because of the trauma they had suffered.
Officers are debriefing the women at accommodation provided by the charity, slowly building up a picture of a life where their every move is claimed to have been scrutinised and any contact with the outside world subjected to control and supervision. The house where they were allegedly held captive was being searched.
The case was immediately compared to that of Josef Fritzl, the Austrian man who kept his daughter captive for 24 years and fathered seven children with her. However, these three women are not related and officers said they had no information that the youngest was related to the two people under arrest. Police emphasised that they were not investigating any allegations of sexual abuse.
The Independent understands that although the women were not kept permanently in the house, their access to the outside world consisted of being allowed to hang out laundry and occasional shopping trips while accompanied by one of their alleged captors.
Aneeta Prem, a magistrate and founder of Freedom Charity, which helps victims of forced marriage and "honour" violence, said that after the initial call, welfare workers and police were able to identify the women's home and set up clandestine meetings.
It is believed the women suffered physical as well as mental harm. Ms Prem told Sky News: "We started in-depth talks with them when they could; it had to be pre-arranged. They gave us set times when they were able to speak to us. It was planned that they would be able to walk out of the property. The police were on standby."
The charity said the women had described their claimed captors as "heads of the family" and were living in a state of terror. Ms Prem said: "They felt they were in massive danger. I don't believe the neighbours knew anything about it at all. It was just an ordinary house in an ordinary street."
Police said they did not yet know if the 30-year-old had been born in the house but it was clear it had been her home ever since.
The Independent understands that the initial caller spoke of a "friend" and did not immediately identify herself or the other two women as victims of domestic slavery.
It is thought that the woman did not give names or an address. The caller, who was in an emotional state, said that the "friend" believed she had suffered a stroke and knew she was not well, but she was not allowed medical help. The caller said the "friend" had had no education at the address.
In a report, the Centre for Social Justice said that in 2012 about 800 adults were known to have been enslaved in the UK, but it said this figure was a "pale reflection" of the true scale of the problem.
Adults and children were exploited through the sex industry, domestic servitude and enforced criminal activity. The charity said its research showed that many cases were never recognised or reported and that it had no consistent grip on the numbers. "Agencies charged with such responsibility are groping in the dark for a sense of scale," it said.
"They felt they were in massive danger. I don't believe the neighbours knew anything about it at all. It was just an ordinary house in an ordinary street.," said Ms Prem.
"They were very restricted on everything they could do.
"We are absolutely thrilled this has happened."
Detective Inspector Kevin Hyland, from the Met's human trafficking unit, said: "We've established that all three women were held in this situation for at least 30 years. They did have some controlled freedom.
"The human trafficking unit of the Metropolitan Police deals with many cases of servitude and forced labour. We've seen some cases where people have been held for up to 10 years but we've never seen anything of this magnitude before."
Mr Hyland said there was a delay in arresting the suspects, who are both non-British, after the women were freed on October 25 as police tried to establish the facts of the case.
"The women were released as soon as possible," he said.
"There was a delay in the arrest. This was down to the fact that we had to work very carefully with these people who were highly traumatised and it was very difficult to establish the facts.
Asked about what help the women will be given, the charity founder said: "They are going to be afforded all the help and support that can be.
"I'm so grateful they saw the news. Now they will try to re-build their lives."
Earlier this year the Forced Marriage Unit (FMU) - a joint operation by the Home Office and the Foreign and Commonwealth Office - revealed it helped in 1,485 cases of possible forced marriage in 2012, involving 60 countries across Europe, Asia, the Middle East, Africa and North America.
- The Independent
