You can expect "trouble" and social unrest among London's poor if the capital's public transport does not improve, the head of Transport for London (TfL) has said.
Without major investment, the overcrowded network will be unable to transport people to work as London's population grows and the city would be left "in a bad way", Sir Peter Hendy said.
He said "the stakes are pretty high" and unrest would happen if transport became too crowded for people travel to work.
"London's poor don't live in Harrow Road, they live in Enfield and Tolworth and if you can't get them to jobs they want, your city's going to be in a bad way: it's not going to progress and contribute to national economic growth," he told The Guardian.
"The stakes are pretty high. If you're not able to increase transport capacity, and people find accessing work impossible, you risk social unrest. You can expect trouble."
He said Crossrail - the new east-west route through Greater London - would be "full within months" and second one would have to be built very shortly afterwards.
"You won't be able to do without it. In central London the overcrowding will grow to be overwhelming," he said.
London's population has surged to around 8.4 million, according to the most recent estimates, after decades of decline.
If the trend continues, the city will have 10 million people by 2030, growing by around 80,000 a year.
Hendy said the overcrowding could lead to a situation similar to that in Mumbai.
The Indian city has seen demonstrations against the congestion and steep increases to rail fares this year.
Some fares nearly tripled in some cases.
"The mad rush is a daily sufferance...at least one passenger falls off every day while trying to board it. We are injured, bruised," one commuter told The Times of India.