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Telefonica is launching an app that allows smartphone users to make calls and send messages without using up their quota of call minutes or texts.

Actions taken by "Tu Me" will instead be deducted from data allowances.

It poses a challenge to existing apps including Skype, Viber and Whatsapp.

Network operators have traditionally viewed such programs as a threat, but the owner of O2 said it would "rather keep the customer than lose them to other products and services".

Tu Me is free to download and will be promoted to members of Spanish telecom Telefonica's O2, Movistar and Vivo networks. The company has about 300 million customers across the world.

However, subscribers to other brands will also be able to use the program.

Both the caller and recipient must have the app installed for it to work. Users will receive a pop-up notification when someone is trying to get in contact, prompting them to launch the software.

Unlike Skype, users cannot make calls to normal telephone numbers.

Photographs, location information and voice messages can also be sent with all data stored on Telefonica's servers.

The company said messages would be encrypted when transmitted and it promised not to analyse or provide third-party access to the contents unless required to do so by the courts.

"We've seen the growing popularity of communication apps on smartphones but we believe we've gone one better with Tu Me using our knowledge and insights of how people use their devices," said Telefonica Digital's chief commercial officer Stephen Shurrock.

The service will initially be limited to Apple's iPhones, with a version to follow for Google's Android system.

Telefonica stands to benefit if its subscribers increase their data allowance, although the service can also be used via wi-fi.

"This has been well thought through as a response to Skype and other voice over internet services," Jeremy Green, principal analyst at the technology research firm Ovum, told the ³ÉÈË¿ìÊÖ.

"It's a bit of a case of if you can't beat them join them. The tweak on it is that the service is still anchored to the users' telephone number and from a telecom firm's point of view that's a good way to protect their revenues."

A recent study by Ovum suggested that social messaging apps had cost network operators a total of $13.9bn (£8.6bn) in lost SMS revenues in 2011.

Telefonica said that it planned to add functionality to the app and launch further products over the coming months.