Martin Lewis has issued an eight-week warning to mobile phone users ahead of huge bill hikes.
Many of the major telecoms giants are about to put up prices by 14% from the end of March or beginning of April.
Martin explained that up to nine million people might be out of contract and overpaying on their current deal if they do not check to see whether they can get a better price elsewhere.
He stressed the importance of shopping around and using comparison websites for the best deals.
The financial guru issued his warning during his ITV Martin Lewis Money Show broadcast on Tuesday.
He said: "Last week I told you broadband and mobile bills are going up 14% in April for most of the big firms.
"What I want you to do first is I want you to check if you're out of contract. Over nine million of you are overpaying."
He added: "If you're out of contract there are actually some of you - outrageously, and this should not be allowed - who got a contract with a handset and once you paid the handset off, they keep the price at the same level so you're still paying the with-handset price even if you're no longer using it."
The money expert continued to explain what you should do if you change networks but wish to keep the same number.
On the show last night, he explained that all the providers are just "piggybacking" on their signals, so it is important to do a comparison search before settling with a single network.
He said: "There are only four networks in the UK - EE, O2, Three and Vodafone. Every other mobile provider is piggybacking on their signal.
"Some comparison sites will let you do a comparison limiting to the signal that you get, so you may be able to save money and stick on your signal.
"I'm not saying it will be exactly the same service - these are different firms, you might not get WiFi calling, you might have to pay for voicemail, you might not get 5G, you might not be able to tether.
"But the point is, if you're staying where you are solely because you like your signal, why not give one of the piggyback providers, if they're cheaper, a go as well?"
How to cut the price of your mobile phone bill
If you’re out of contract, chances are you’re paying too much for your mobile phone.
Compare prices elsewhere using a comparison website to see if you can get a better deal.
When comparing prices, go back and see how many minutes and data you're using in your current deal so you know what type of package you need.
The cheapest phone tariffs are typically SIM-only deals.
This is because you're only paying for your minutes, data, texts and any other allowances - not the handset.
Don’t want to switch elsewhere? Talk to your current provider, explain the prices you’ve seen elsewhere and try to haggle them down.
Do keep in mind that if you’re in contract, you may be charged an exit fee for switching.
