A street cleaner at work in San Francisco, where homelessness is soaring and has reached 20 times the rate of London

On a summer morning, a man is hosing down the pavement in front of Old Siam, a restaurant in the Tenderloin district of San Francisco.

“Once in the month, twice in the month, there’s poop in front of my door,” he said. “I have to clean it. Who’s going to clean it up — the city? No.”

San Francisco, the richest city in America, is grappling with the dirty downside of its tech boom: a rash of human “poop” deposited on the streets by thousands of homeless people who have nowhere else to go.

The deluge of wealth enjoyed by well-paid techies has pushed the median rent for a one-bedroom apartment to $3,720 (£3,067) a month — the highest in America. The median price