For each person sleeping rough on London’s streets, there are 13 more “hidden homeless” who sofa surf, sleep on buses, squat or have sex with strangers each night, a report said on Wednesday.

The London Assembly — an elected body that holds London’s mayoral office to account — said more than 12,000 people do not have a home, often risking assault or abuse, but are excluded from official statistics and cut off from support.

“People sleeping on the streets of our city are just the tip of an iceberg,” Sian Berry, chairwoman of London Assembly’s housing committee, said in a statement.

“So-called sofa surfing is common and people can end up staying with virtual strangers where they are vulnerable to exploitation and abuse.”

The report said young people are especially likely to be affected, particularly lesbian, gay, bisexual and transgender (LGBT) people, and those who have experienced domestic violence.

Some 225,000 Londoners ages 16 to 25 have stayed somewhere unsafe because they had no home to go to, it said, with some resorting to “survival sex” to get a bed for the night.

“Young people, asylum seekers and people escaping domestic violence can find it hard to get help due to gaps in current policies, and many don’t even try to seek help,” Berry said.

Rents in London are among the highest in the world, with the average home costing more than £1,800 ($2,400) a month, according to property lender Landbay.

Many young homeless people who would be eligible for housing support do not receive it, because they do not seek help and are not recognized as vulnerable by social workers, it said.

Only one in five young hidden homeless contact their local government authority and one in five of those who do reach out are turned away without receiving assistance, it said.

In April, a law will come into force in Britain requiring local councils to prevent homelessness by supporting anyone in danger of losing their home. Critics say the government has not made clear how councils can afford to fulfill their new duties.