Justin Gutmann is the former head of research and insight at Citizens Advice. As a consumer champion, he has previously undertaken class legal actions against Apple and UK train operators. Below, he explains how he is taking on the telecoms giants.

There's nothing wrong with taking out credit. We all do it to pay for big purchases, whether that's buying a home or a car. 

We enter into contracts, take possession of the item and once we've paid it off, we aren't charged any more money. But there's one big exception - mobile phone contracts.

For decades, the UK's biggest mobile network operators have been selling us contracts made up of mobile phone and airtime services like data, calls, and minutes. 

When it comes to the end of the contract, we've already paid for the phone. Yet companies like Vodafone, EE, Three, and O2 kept charging us the same price.

This is what's called a 'loyalty penalty'. Existing customers are charged for something they have already paid for.

My claim is that overpayments to Vodafone, EE, Three and O2 come to a total of over £3billion pounds in extra charges. 

And last Friday, I launched a class action to try to get that money back on behalf of millions of their customers.

Three billion pounds may sound like a lot of money - the amount people have been overpaying quickly adds up. 

You can read the full article on the Daily Mail website here.