The UK's smallest mobile operator Three has gained almost a million customers over the last 12 months, according to figures reported in its parent company's latest earnings call.

Compared with H1 2011, Three’s registered customer base is up 13% to over 8.4 million. Meanwhile its most profitable segment – contract customers – has grown by 21% in the past year.

Dave Dyson, CEO of Three UK, said that the growth had been supported by strong growth in the smartphone side of the market, and increased demand for unlimited mobile data services.

“Smartphones are incredibly powerful devices that can really do a lot for customers, and customers want to get the most out of that opportunity. To do that they need a very good network and they need a tariff that allows them to use their smartphone without the fear of running up a big bill,”  he said.

“For the last five years we've invested very significantly in the quality of the network. We still are the only operator that has a network that is solely built for the internet, and we will continue to focus in on network quality.”

Three's parent company, Hutchison Whampoa, revealed that the company's earnings before interest and taxes (EBIT) were £26 million in the first half of 2012 – almost double that of the equivalent period in 2011.

“I think the key message we'd like to put across is we are managing what has been a significantly accelerated period of growth in terms of acquisition, while maintaining the bottom line profitability,” said Richard Woodward, CFO of Three.

Last week, mobile operator Three announced that its four million contract mobile phone customers have more than doubled their monthly data use over the last year. The average contract customer uses 1.1GB of data each month, up from 450 megabytes last summer.

Dyson said that this trend would continue to grow, provided that operators can give good value tariffs, good quality network experience and give customers peace of mind that they are not going to rack up significant out of bundle charges.

Unlike some mobile operators, which cap the amount of data their customers can use, Three offers unlimited data packages as part of its One Plan. Dyson said that Three is working with T-Mobile to ensure that its network scales in line with demand.

“Our network is standing up to every test that our customers are throwing at it,” he said.

Overall, Hutchison Whampoa's first-half profits beat analysts’ estimates, rising 13%. The company said that, in spite of global economic woes, earnings were helped by investments in electricity, water and gas networks, and demand for services from British smartphone users.