Millions of mobile phone customers on the Three network face a huge rise in charges for international calls and texts and for UK calls that are over their monthly allowance.

From 18 June, customers making international calls from the UK will face some price rises of more than 100%. Calls to European countries will rise from between 46p and 56p a minute to a flat charge of £1.25 a minute.

Calls to other parts of the world from will cost £1.75 a minute – up from the 46p-£1.02 a minute Three currently imposes.

Pay monthly customers who have exceeded their allowance will see the cost of calls to UK landlines and other mobiles rise from 35p a minute to 55p a minute – a 57% increase.

The cost of sending an international text will rise to 35p – up from 25p now. Calls to premium-rate numbers – those beginning with 084 and 087 – rise from 45p a minute to 55p.

Pay-as-you-go customers will not be affected by the changes.

The company, which has 9.9m customers in the UK, reported revenues of £2.4bn in 2017 with an average margin per user of £12.04, said it was the first time it had increased such charges in a number of years.

“We are continuing to make investments in our network to offer the best possible experience for our customers,” it said.

All the mobile providers have been hit by the abolition of EU roaming charges by the European Commission, and this increase looks to be a way for the company to replace that income.

The new price rises come on top of a 4% increase in Three’s contract charges, which took affect in February. There were similar rises from the other big mobile providers.

While Ofcom rules let customers escape penalty-free if monthly tariffs increase and they were not warned at sign-up, the same protection does not apply to out-of-allowance price increases, unless users can claim they have suffered “material detriment” as a result. That is defined as a rise that is bigger than the retail price index rate, which is currently 3.3%.

Three says that for most customers the increase is unlikely to be a “materially detrimental change”.

A spokesperson said: “Regular users of outside-of-allowance calls who are likely to suffer material detriment as a result of the price increase will be given the right to leave without charge, in line with our terms and conditions. We expect that this will be less than 1,000 customers.”

The price rise affects all monthly customers on handset plans that joined Three after March 2014, those on sim-only plans that joined after 15 July 2014, and anyone on a mobile broadband plan that joined since August 2015.

In the company’s last financial report, its chief executive, Dave Dyson, stated Three’s commitment to “becoming the best-loved brand by our customers”.

Ernest Doku, the mobiles expert at uSwitch.com, said: “Three are within their right to implement these increases; however, the usual Ofcom rule of a 30-day period where you can leave penalty-free does not apply as these increases are for out-of-allowance charges.

“It is especially important that consumers don’t get confused between chargeable international calls and fee-free roaming they might get with their tariff – else they could be stung with a massive bill.”