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Mobile network Three has defended its decision to end a popular "all you can eat" phone contract.

Thousands of customers currently paying £17 a month for unlimited data and calls have been told they will be moved onto a new £30 tariff if they do not opt out within 30 days.

The company has notified customers by post and said it would also text them.

Three said the more expensive plan was the closest remaining deal offering unlimited calls and data.

It stopped offering the £17 monthly deal to new customers in 2014 and said it was phasing out "legacy" tariffs, but the ³ÉÈË¿ìÊÖ understands hundreds of thousands of customers still use the tariff and will be affected by the switch.

One mobile industry analyst told the ³ÉÈË¿ìÊÖ the demise of unlimited data plans was "inevitable".

"Consumer data usage is growing exponentially," said Ben Wood from CCS Insight.

"The networks are seeing huge growth in data consumption as people watch more video content at ever-higher resolutions on their smartphones. At some point certain all-you-can-eat tariffs become uneconomical."

While the network does still offer plans with unlimited data and calls, Three says its average account holder consumes just 4.9 gigabytes of data per month.

The firm had 8.8 million customers in 2015, according to its website.

Some have posted their anger at the tariff change online, ironically tagging their posts #MakeItRight - the hashtag Three uses in its advertising campaign.

"That's how you lose brand loyalty," .

"Stop forcing loyal customers like myself into new plans which are almost double the price," .

In a statement, Three said: "In March 2014, we introduced new price plans giving customers more options in the size of their data and voice bundles, as well as limits and alerts to prevent bill shock.

"We have a lot of tariffs that we no longer sell and moving customers to one of the new plans will ensure they can enjoy the benefits of these plans."