A group of private investment firms is to pay more than $7bn (�4.58bn) for Qwest's directory publishing unit.

QwestDex is known for its high profit margins and steady revenue stream from the distribution of 45 million directories in 14 US states.

"The sale of QwestDex is a significant part of our plan to deliver and strengthen our balance sheet and will allow us to focus on maximizing the profitability of our core operations," said Qwest's chairman and chief executive, Richard Notebaert.

A private equity consortium led by Carlyle Group and Welsh Carson Anderson & Stowe sealed the purchase following four months of competitive bidding.

The price was lower than the $8-10bn which analysts had predicted, but will help Qwest to clear some of its debt mountain of more than $25bn.

Investors reacted to the news by snapping up Qwest shares, which soared more than 27% during the first half hour of trading.

Troubled parent

Since the bidding for QwestDex begun, its parent Qwest has said it would restate its results for the last three years, and posted a $1.1bn loss for the April to June period.

In addition, Qwest's accounts are under both criminal and regulatory investigations in the US.

The deal would provide the troubled US telecoms firm Qwest with a rapid and much needed $2.75bn cash injection which should help the firm service its debts.

The remaining $4.3bn should be paid in cash later on.

Qwest has held talks with its creditors to renegotiate some loans in an effort to beat off the threat of bankruptcy.

Qwest shares closed $0.31 to close at $2.24 on Monday.