Twitter vows to be more transparent with its users, as Apple says that apps which collect user data violate the company's guidelines for developers

Twitter says it will make privacy guidelines more explicit after it admitted copying entire address books from people's smartphones and storing the information on its servers, according to reports.

Both the micro-blogging service and Apple, whose iPhone offers the Twitter app, have come under fire following the revelation.

Reports say that two US congressmen have written to Apple asking why the firm allows the practice on its iPhone, as it contravenes app developer guidelines.

Twitter spokeswoman Carolyn Penner told the BBC and Los Angeles Times: "We want to be clear and transparent in our communications with users.

"Along those lines, in our next app updates, which are coming soon, we are updating the language associated with Find Friends to be more explicit."

She also said that in place of "scan your contacts", the site will use "upload your contacts" and "import your contacts" instead, for iPhone and Android apps respectively.

A spokesman for Apple said: "Apps that collect or transmit a user's contact data without their prior permission are in violation of our guidelines.

"We're working to make this even better for our customers, and as we have done with location services, any app wishing to access contact data will require explicit user approval in a future software release."

The copying of address book contacts was made evident when an app developer in Singapore noticed that his contacts had been copied from his iPhone without his consent by a social network called Path.

On his blog, Arun Thampi spoke of the widespread attention his observation had received.

He wrote: "As an aside – never in my wildest dreams did I imagine this to blow up like this. I hope we can keep calm and continue to discuss this sensibly."

In a blogpost, Path chief executive Dave Morin said: "We used the data for the sake of simplicity.

"Any time you build a network, you have to help users find their friends. And that entire experience is designed to suggest people who you're close to."

The Los Angeles Times reports that the Twitter app stores the full iPhone contact list for a period of 18 months.