The arrests this week of two people in London accused of keeping three women as slaves for more than 30 years is shocking but sadly, not unusual, says one expert in the field.

On Thursday, London police announced they had arrested two 67-year-olds in the south London neighbourhood of Lambeth on suspicion of holding three women as slaves -- a 30-year-old Briton, a 69-year-old Malaysian and a 57-year-old Irish woman –for three decades.

Police have remained tight-lipped about the case, refusing to reveal if the suspects were a couple nor the suspected motive. They initially said they did not believe any of the victims were related, but later said the relationship between the three is still part of the investigation.

While there is no evidence yet to suggest anything of a sexual nature, police cautioned the investigation is still not finished.

Jakub Sobik with the group Anti-Slavery International says the case is yet another example that slavery continues in the modern day, even in developed countries.

"It's quite a shocking case, but unfortunately, not surprising. We have been all too aware of this problem," Sobik told CTV's Canada AM Friday from London.

"We estimate there are more than 20 million people in slavery right now across the world and no country is exempt from that."

The arrests came about last month after one of the older women phoned a group called Freedom Charity to say she and two friends were being held against their will. It took another week of contact with the women to convince them that it was safe to leave the house and accept rescue.

The two suspects were arrested on Thursday after police finished questioning the women. The suspects are reportedly not British citizens.

One of the older women was the one who decided to call for help. She and the others had recently watched a televised documentary about forced marriage that focused on the Freedom Charity. She then used the toll-free phone number shown at the end of the program to contact the charity to ask for help.

The women have told police they were help inside the home for three decades and forced to perform domestic tasks, stepping outside only to hang out the laundry. The youngest woman is believed to have spent her entire life in servitude. It's unclear where she was born.

Det. Insp. Kevin Hyland, the head of London Metropolitan Police's human trafficking unit who is leading the investigation, said the women’s alleged captivity and forced slavery was three times longer than anything ever seen before by police in the country.

Sobik says it's difficult if not impossible to estimate the size of the slavery problem around the world.

"In the U.K. last year, there were over 1,000 cases registered as potential victims of human trafficking. This is an official number but obviously, the estimate of real numbers is quite a bit higher because it's very much a hidden phenomenon," he said.

He says this case shows that no country is free from modern slavery.

We like to think these problems only happen in underdeveloped countries, maybe in Asia or India or Africa. But this case shows that no one is exempt from slavery, even people from the U.K., he says.

Sobik says victims of slavery are obviously traumatized and need all the support and services they can get.

"We hope they’ll get to enjoy their freedom as soon as possible, but clearly, there's much work and support that they will need," he said.

The women, whose names have not been released, are now being held at an undisclosed location in Britain and have been working with severe trauma experts since their rescue, Hyland said.

He said the women are "highly traumatized," having had "no real exposure to the outside world" for the past 30 years, and that trying to find out what happened over three decades will take some time.

With reports from The Associated Press