A policeman planned to "have words" with a driver who had just overtaken him but instead found himself helping deliver the man's baby daughter.
PC Dan Wells was driving along when a BMW raced past him and pulled over.
The driver, Russian banker Michael Nadel, 39, flagged down the police car in Fulham Road and told the officer and his partner that his wife Natalia, 31, was about to give birth.
PC Wells called an ambulance but Mr Nadel, from Surrey, told him his wife's labour was moving faster than expected.
When PC Wells and his colleague went over to the Russian couple's car, the constable noticed the baby was turning blue and moved the umbilical cord which was wrapped around the baby's neck, restoring her oxygen supply.
"I didn't know what to do but I told the mum to push and out came the baby," he said.
Mr Nadel praised PC Wells for helping to deliver Nikol, who weighed 6.4lb when she arrived at 3.25am yesterday.
He said, "The policeman really helped us. I'm very, very happy."
PC Wells, who has no children of his own and has had no training in delivering babies, said: "I just did what I thought at the time was right."