Motorists have been warned that a common headlight habit could see them hit with a hefty fine. Breaking this little-known rule could result in a penalty costing as much as £1,000.

Many drivers will flash their headlights from time to time. It can be used as a way to say "thank you" to other drivers or to warn of an upcoming obstacle.

However, it comes with its risks as it can be seen as "obstructing" a police officer's duty if you flash your headlights to warn of a speed camera. This could see you fined between £30 and £1,000 if caught.

Halfords explains: "If you're caught flashing your headlights to passing drivers to warn them about an upcoming speed trap, you could find yourself in a spot of bother.

"While to some, it's an act of goodwill, the law sees it as obstructing an officer's duty. Being caught can cost you between £30 and £1,000."

Driving school Drive Johnson's also highlights this issue. "Flashing your headlights can be used to signal to other drivers that there is a speed camera or police ahead," it says on its website.

"Many drivers are unaware that this helpful gesture can see you facing a fine yourself.

"Motorists can receive fines of up to £1000 for flashing their headlights to warn other drivers."

This is acknowledged in the Highway Code, under rule 110. The code says: "Only flash your headlights to let other road users know that you are there. Do not flash your headlights to convey any other message or intimidate other road users."

According to the Department of Transport, it could also be seen as a breach of section 89 of the Police Act 1997. This law states that an offence is something that tries to "willfully obstruct a constable in the execution of his/her duty".

Drive Johnson's continues: "Therefore, police could charge anyone who appears to be obstructing their ability to conduct speed checks on other motorists by warning them. The maximum fine for obstructing a police officer is a penalty of £1,000."

As stated in the Highway Code, the correct time to flash your headlights is when you are alerting other road users to your presence.

This means you should only flash your headlights to warn other drivers you are there if you are in a dangerous position or you think they may not have noticed you.