Martin Lewis says he's becoming 'increasingly concerned' about a new payment scheme.
The financial guru was speaking on BBC Radio's Ask Martin Lewis podcast about buy now, pay later schemes being offered for online purchases.
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He said: "There is a regulation that was originally set up so if you're a gym or if you're a furniture shop you can offer zero per cent credit for a year and you don't have to be regulated.
"But what's happened over the past few years is firms have sprung up that tend to come from highly regulated countries and have moved into the UK where effectively they are unregulated for this buy now, pay later activity.
"We've seen it explode over this year...and you're always offered these buy now pay later options from retailers, sometimes people aren't even aware they are opting for buy now pay later.
"And it's something I'm getting increasingly concerned about..."
Martin added: "My big issue is this, this is an explosive form of credit that is becoming very quickly dominant and it's not regulated.
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"That means it's not controlled properly and more importantly when things go wrong you don't have a right to go to a financial ombudsman so I think I would put a warning out there.
"Think carefully, make sure you understand what you're doing when you buy now pay later.
"Make sure you understand it's credit, it's a form of debt- make sure you know the rules or don't touch it."