Britney Spears is smoking ... we're not talking about her looks, we mean she's sucking on cigarettes in a dangerous locale -- the gas station!

Photogs caught the Britney Spears ''Circus'' as it came to the local business ... with her driver pulling her Mercedes up to the pump.

While this dude hops out to fill her up, Britney ashes her cigarette out the window ... in full view of a "No Smoking" sign.

As we're sure you know ... gasoline and fire -- not the best of pals ... but, thankfully, Britney came out of this one untorched.

Of course, Spears isn't driving herself because she was booked for DUI last month. CHP told us she was arrested for "driving under the influence of a combination of drugs and alcohol."

She's most kept a low profile in the weeks since -- sticking to her home in the first few days after the arrest and rebuilding her relationship with her kids.

Still, maybe ash those cigs Brit ... at least when there's flammable material around!