Under pressure from US legislators, Apple has moved to quell a swelling privacy controversy by saying that it will begin to require iPhone and iPad apps to seek "explicit approval" in separate user prompts before accessing users' address book data.

Apple's move came shortly after two members of the US House Energy and Commerce committee requested the company to provide more information about its privacy policies. Bloggers, in recent days, have published findings that some of the most popular software applications in Apple's App Store have been able to lift private address book data without user consent.

iPhone 4.

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

In a letter addressed to Apple chief executive Tim Cook, Representatives Henry Waxman of California and G.K. Butterfield of North Carolina, both Democrats on the House Energy and Commerce Committee, asked Apple earlier on Wednesday to clarify its developer guidelines and the measures taken by the company to screen apps sold on its App Store.