Twitter has said it will also update its apps for iPhone and the Google Android operating system to make it clear that emails and phone numbers are uploaded to its servers, where they may be kept for 18 months.

Contacts unlocked? It has emerged that some iPhone
apps take copies of the user's address book.
Photograph: Carolyn Jenkins/Alamy

The software overhauls were announced after two members of U.S. Congress wrote to Apple chief executive Tim Cook demanding answers to "claims that the practice of collecting consumers' address book contacts without their permission is common and accepted among [third-party] app developers."

In response, Apple said any apps accessing address books without prior permission were in breach of guidelines and would be stopped in their tracks by forthcoming software upgrades. "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," it said in a statement.

Last week, it emerged that Path, a social networking app, uploaded and kept a copy of new users' address books without consent. One developer, Dustin Curtis, wrote: "I did a quick survey of 15 developers of popular iOS apps, and 13 of them told me they have a contacts database with millions of records. One company's database has Mark Zuckerberg's cell phone number, Larry Ellison's home phone number and Bill Gates' cell phone number. This data is not meant to be public, and people have an expectation of privacy with respect to their contacts."