You may use front fog lights with headlights ONLY when visibility is reduced to less than

Explanation: It is an offence to use fog lights if the visibility is better than 100 metres (328 feet). Switch front fog lights off if the fog clears to avoid dazzling other road users, but be aware that the fog may be patchy

Category: Vehicle Handling

References: The Official DVSA Guide to Driving: Section 12, Highway Code: Page 128