The coronavirus pandemic has seen many retailers, hospitality providers and other businesses switching to card-only transactions and refusing to accept cash.
Some people believe the move is illegal, saying that businesses in this country have to accept any legal tender.
Answering the question on whether businesses are breaking rules by demanding card payments on ITV's Martin Lewis Money Show, he said: "No, they are not breaking any rules."
"You are allowed to take card only as long as it's not discriminatory for race or disability or something."
Martin then explained what 'legal tender' means. He said: "The legal tender thing is interesting.
"Legal tender has a strict definition. It means if you have a court awarded debt against you, if someone tries to settle and they're paying in legal tender you cannot refuse it.
"And that's all it means."
He pointed out that in some parts of the country, the only thing that's legal tender is, in fact, coins.
"In Scotland, no notes - neither Bank of Scotland nor Bank of England, are legal tender."
But that doesn't mean they're not legal.
"They're all legal currency," he added.
