A 69-year-old Malaysian woman, a 57-year-old Irish woman and a 30-year-old Briton who had apparently spent her entire life in servitude were rescued last month after calling a charity, police said.
Police arrested a man and a woman, both aged 67 and described as non-British nationals, at the house in south London yesterday but the pair were freed after questioning.
"Two people arrested on Thursday, 21 November, in connection with an investigation into slavery and domestic servitude have been bailed until a date in January pending further enquiries," a police statement said.
"We have never seen anything of this magnitude before," said Detective Inspector Kevin Hyland, who leads Scotland Yard's Human Trafficking Unit.
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"These women are highly traumatised, having been held in servitude for at least 30 years with no real exposure to the outside world, and, trying to find out exactly what has happened over three decades will understandably take some time," he told reporters.
Police said there was no evidence to suggest that the case involved sexual abuse.
Detectives said they did not know where the youngest woman was born, adding that the relationship between the three women was "part of an ongoing investigation and we are not willing to speculate".
The women, who are now in an unspecified location, were rescued after the Irish woman "found the courage" to call the Freedom Charity on October 18 after it was featured in a television programme about domestic slavery, police said.
The charity usually deals with forced marriage and honour-based abuse but is also used to working with women who feel trapped in difficult situations.
The Irish woman said she and two others had been held against their will in a house in London for 30 years.
Police identified the address where they had been held, in the south London borough of Lambeth, and went there to rescue the Malaysian woman.
The two arrests were made almost a month later "as soon as officers had established the facts and were satisfied the victims were aware of police intentions", the police said.
The three women had "controlled freedom", but Hyland said it was still not clear what that meant in practice.