By Ben Edwards

Riskier companies are issuing debt at their fastest-ever clip in Europe, and now the market looks set to welcome its biggest offering on record.

Monday, French cable operator Numericable Group SA started meeting with investors as it prepares to raise an equivalent of about 6 billion euros ($8.33 billion) from three chunks of high-yield, or junk, debt, denominated in both dollars and euros.

Demand for high-yield bonds has remained rampant in Europe this year, reflecting record low interest rates that prompt investors to load up on riskier debt that offer chunkier returns. Junk-rated companies have raised about EUR17 billion from euro-denominated deals since January, the most at this stage of the year on record and almost a third more than at this time in 2013, the previous fastest start to a year, according to Dealogic.

That demand is also allowing companies with sub-investment grade credit ratings to snag their lowest ever borrowing costs. The average yield for euro-denominated junk bonds hit 4% last week, a record low, according to a Markit index.

"It's a very favorable environment to issue debt," said Arnaud Tresca, head of debt capital markets for high yield at BNP Paribas in London.

Numericable--a subsidiary of Luxembourg-based telecommunications group Altice SA--is seeking to raise funds to help finance Altice's EUR17 billion acquisition of Vivendi SA's telecoms business SFR.

Altice, controlled by cable-tycoon Patrick Drahi, earlier this month won a fierce bidding war over SFR, France's second-largest mobile operator.

Altice offered to pay EUR13.5 billion in cash when the deal closes, plus a potential further payout of EUR750 million. On top of the cash payment, Vivendi will also hold a 20% stake in a new group formed by merging Numericable with SFR.

Separately, Altice is planning to raise an equivalent of EUR4.15 billion from an eight-year bond sale, proceeds of which will be used partly to buy shares issued by Numericable to help finance the SFR purchase.

Numericable's debt offering will eclipse the $5.7 billion raised by NXP Semiconductors Netherlands BV in October 2006, the previous biggest European junk bond sale on record, according to data provider Dealogic.

The Numericable bond is expected to be rated Ba3 by Moody's Investors Service, three notches below investment grade. Numericable Group's long-term corporate credit ratings of 'B+' were affirmed Monday by Standard & Poor's Ratings Services, a rating that is four notches below investment-grade. S&P also gave Altice SA a preliminary long-term corporate rating of 'B+' on Monday.

Altice and Numericable plan to meet with investors in London and the U.S. this week, before concluding in Paris and Frankfurt on April 22.

Deutsche Bank, Goldman Sachs and J.P. Morgan Chase & Co. are coordinating the bond sales.

Ruth Bender contributed to this article.

Write to Ben Edwards at ben.edwards@wsj.com

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