The foul smell from a leak at a chemical plant in northern France has been reported in Bedfordshire and Hertfordshire, police said.
People in Kent and Sussex first noticed an odour, compared to that of rotten eggs, on Tuesday morning.
By the afternoon, the smell had been reported in places including Welwyn, Wheathampstead and Leagrave.
The Met Office said the wind speed and direction made it "entirely possible" for the smell to have spread that far.
The smell has been identified as that of mercaptan, the chemical added to natural gas to give it an odour.
It escaped from a factory near Rouen 75 miles (120km) north-west of Paris on Monday.
A Met Office spokesman said Tuesday's light winds moving in from the east and south east meant conditions were "certainly conducive" to it reaching Bedfordshire and Hertfordshire.
He added that on Wednesday, winds were still generally blowing in from the east so it may still be detected.
Hertfordshire Police said they had had "a few calls" about the smell, but had not issued any specific advice or warning.
"Obviously we wouldn't want people to ignore it but people should call the fire brigade or the National Grid if they believe the smell is coming from their own home," a spokeswoman said.
Bedfordshire Police have also put out Health Service Agency (HSA) advice on its Facebook page.
The HSA said the smell poses no risk to public health.
"It is not toxic and has also been diluted before entering the air over England, so people should be reassured it will cause no harm," a statement read.