Is it illegal to charge a surcharge on a debit card?

Could you please clarify for me if it's actually illegal to impose a surcharge on debit card transactions? I've heard conflicting opinions on this matter, and I'm curious to know if there's a definitive answer. Is it a violation of any laws or regulations to charge a customer an extra fee when they pay with a debit card? If so, what are the specific consequences for merchants who engage in this practice? I'd appreciate your insights on this matter.