Motorists in Westminster who sit in a stationary car with the engine running and refuse to turn it off are to be fined £20.
The move reflects concerns from local residents and businesses about air quality, Westminster City Council said.
A team of traffic marshals will patrol the borough and ask car owners with stationary vehicles to switch off their engines, the council said.
The penalties will come into force on 1 May, as a “last resort”.
The new policy follows Camden Council’s fines for buses which idle for too long, introduced in 2011.
Neil Greig, director of policy and research at the Institute of Advanced Motorists, said the “key issue” would be how the marshals enforced the fines.
He said: “If they target private car drivers pulling up for a moment in relatively clean, modern cars in a draconian way – whilst ignoring old buses, coaches, large trucks, utility company vans or taxis idling for long periods – then it will quickly be seen as yet another revenue-raising exercise.”