Crack cocaine is being used by up to 46,000 Londoners according to new research, a much higher figure than previously thought.
The highly-addictive drug is relatively cheap
A report out this week suggests one in 100 Londoners aged 15 to 44 is using the highly-addictive drug.
One of the authors, Matthew Hickman from Imperial College London, said: "We must be cautious, but analysis suggests there is a substantial problem."
Research was based on data from drug centres, hospitals and police arrests.
Addicts often live chaotic lifestyles, committing crime to fund their habits while crack houses can bring threatening behaviour to neighbourhoods.
Mr Hickman added: "Although crack cocaine use has been a cause for concern in many countries since the 1980s, there has not been the predicted epidemic across the UK until now."
The study by researchers from Imperial College and Bristol University was published in the Society for the Study of Addiction.
"Kaz", a recovering addict who has struggled with drugs for 25 years, said crack addicts could be seen all over London.
He told BBC London: "Any little street corner, any little hole that you pass, they are in there, smoking.
"They are even smoking in the streets in broad daylight, walking with their pipes in their hands, even at bus stops."
Ranjani Utpala, from The Harbour - a treatment centre for crack dependence in Coldharbour Lane, Brixton - said: "The focus for too long now has been on the mainstream drug users, which are the opiate users.
"I think, because of the increase in people using crack cocaine, there's a need for services such as The Harbour, that will target the specific needs of crack users."