Environment department Defra said high levels recorded on Tuesday in eastern England and the Midlands would spread.
The pollution – a mix of local and European emissions and dust from the Sahara – is set to move across southern England, the Midlands and East Anglia.
Those with lung or heart disease are urged to avoid strong exercise outside.
Defra has a 10-point scale for measuring air quality – with 1 meaning there is a “low” risk of air pollution and 10 warning of “very high” levels.
Levels hit the maximum of 10 in north-west Norfolk on Tuesday.
Readings for Wednesday are expected to reach 8 or 9 in much of southern England, East Anglia and the Midlands, and possibly 10 along England’s southern coast.