Three offers unlimited data for £15 per month to pay as you go cheapskates

Three made waves with its One Plan unlimited data tariff at a time when its rivals were reining in their plans for data-hungry smart-phone owners. Now there's an equivalent for its pay as you go customers, starting at £15 a month.

The new plan is called All in One, and is available in two flavours. All in One 15 costs £15 a month and includes all you can eat data, 300 any-network voice minutes and 3,000 texts. All in One 25, meanwhile, bumps the voice minutes up to 500 for £25 a month. Existing pay as you go users can upgrade to the new tariffs by giving Three a bell.

Before you ask, no, the tariff can't be used for the iPad 2 or other tablets with a pay as you go micro-SIM. We asked Three, and the company confirmed All in One is for pre-pay smart phones only. Doh!

All in One sits alongside Three's existing £10 pay as you go top-up, which includes 500MB of data. The operator is offering a competition to promote the launch of the new plan, with prizes including InterRail passes, free groceries and subscriptions to Spotify and LoveFilm for streaming music and movies.

"We want to get people using their smart phones in the way they were designed to be used, and that means data," says sales and marketing director Marc Allera. "More importantly, we want people to feel comfortable doing this without worrying about the cost."

Three says it decided to launch the new plans after finding out many of its pay as you go customers didn't understand how much data was used up by different apps and services, so chose not to use them much to ensure they didn't run out of credit.

It's also a sign, though, that the launch of One Plan may not have persuaded many pay as you go users to switch to a monthly contract, no matter how addicted to Facebook and Twitter they were.

How do the new plans compare with rivals? Vodafone charges prepay customers £1 a day per 25MB of data, or offers a £5 per month Web pack with 250MB. O2 has a Web Bolt-on for £7.50 per month with 500MB of data, although users can pay £10 per month for additional access to Wi-Fi hotspots from The Cloud and BT OpenZone.

Orange's Dolphin pre-pay tariff costs £10 per month with "free Internet", but caps this at 100MB a month. Finally, T-Mobile has a £5 per month booster offering 'unlimited internet', with a fair-use limit of 500MB a month.

What's the best value? If you're using more than 500MB a month in data on a pay as you go contract, Three's new All in One plan looks like the business. If you're using less, it looks a tad overpriced next to T-Mobile's booster.

• For the latest mobile phone contracts and pay as you go deals, visit CNET UK's Mobile Deals site.