It also accounts for about 40% of Bertelsmann's earnings before interest, tax and amortisation - a popular means of analysing a company's profitability.
RTL is often referred to as Bertelsmann's "cash cow" and, while Channel 5 recorded a loss of ￡49m last year, it is expected to break even in 2003, six years after the station went on air.
The sale of Channel 5 would mean RTL abandoning a project that has taken up considerable amounts of money and management time.
Insiders at the pan-European broadcasting group said Bertelsmann was unlikely to order a sale against the wishes of Didier Belens, the RTL chief executive, and his board.
Analysts also said the disposal of Channel 5 would not make strategic sense for either company.
"Given that the channel is becoming more profitable and is gaining audience and advertising share, it has the opportunity to grow much further.
"It plugs the hole in RTL's pan-European portfolio so it seems unlikely it would be disposed of," said a note from analysts at CSFB.
Bertelsmann sources said a strategic review had been launched but this did not mean a "for sale" sign had been hoisted above the Channel 5 building in Long Acre, Covent Garden.
They stressed the group was not about to unravel its costly international expansion plan.
Gunter Thielen, Bertelsmann's new chief executive, is a renowned cost-cutter and efficent manager, who has turned the company's services arm, Avarto, into a highly profitable business.
"Mr Thielen is known for being able to squeeze money out of every company. He made Arvato into an outstanding, profitable business," said one Bertelsmann source.