The London-based private- equity house is thought to have spent recent weeks plotting an approach for the business, which could be worth £500m.

UBM has consistently said there are no synergies between PRN, which distributes corporate announcements in America, and the publishing and exhibitions businesses that make up the rest of the group.

Although UBM is not thought to have actively touted PRN for sale, David Levin, the group’s chief executive, has made it clear he would sell for a good price.

Last week Andy Crow, the company’s deputy chief financial officer, told a business conference in Barcelona: “We have said for all our businesses, there are no sacred cows. If someone wants to come and write a massive cheque, then there you are.”

PRN, largely an American business, has been growing strongly because of increased financial-disclosure requirements imposed on companies. This has prompted analysts to increase their valuations of PRN significantly over the past year.

Apax’s plans to buy PRN continue a recent trend of considerable private-equity interest in business publishing.

Last month Apax itself agreed a bid for Incisive Media while Candover and Cinven have put together a mini-empire in Springer Science and Business Media and tried unsuccessfully to buy Informa. Analysts say the sector is attractive for its reliable cashflows and insulation from advertisers.

The sale of PR Newswire would also form part of a line of disposals at UBM executed by David Levin since he became chief executive in April 2005.

On Friday UBM’s shares closed at 697p.

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