Liberty Global, which owns Unity Media, Germany's second biggest cable operator, joined the race on Tuesday. It tabled an 85 euro a share offer, according to a person familiar with the matter, days after the British mobile company said it was in talks about a deal.

Vodafone wants to buy the cable company so that it can offer TV, fixed line and broadband to more of its mobile customers, while Liberty Global wants more consolidation in one of its best performing markets. Vodafone offered about 81-82 euros a share in cash, sources said last week.

Shares in Kabel Deutschland were trading 3.8 percent higher at 85.6 euros by 9.10 a.m. British time, valuing the group's equity at 7.56 billion euros (6.4 billion pounds).

A Liberty Global deal would be closely scrutinised by German anti-trust authorities, a hurdle recognised by Liberty Global's chief financial officer Charles Bracken last week.

He told a Goldman Sachs cable conference on June 12 that while the industrial logic for consolidation in Germany was compelling, regulatory opposition remained a significant barrier to any deal in the near future, the bank said in a note.

Germany's competition regulator in February blocked Kabel Deutschland's bid to take over smaller Berlin-based cable group Telecolumbus for 618 million euros.

A source familiar with the situation said Liberty has already talked to German antitrust office to discuss the deal, but declined to elaborate.

Liberty Global customers in Germany are in the densely populated German state of North Rhine-Westphalia as well as in Hesse and Baden-Wuerttemberg, while Kabel Deutschland is active in the rest of Germany.

Liberty Global's Unity Media Kabel BW and Kabel Deutschland hold about 15 percent of the German broadband market. With cheaper prices and higher speeds, they have been winning customers from Deutsche Telekom, which still has more than 40 percent of the market.

Vodafone holds roughly 12 percent of the German broadband market, and rents wholesale capacity from Deutsche Telekom to provide the service to customers.

A source familiar with the situation said Kabel Deutschland had requested binding bids in two weeks. The ability of the bidders to fund a deal would also be considered, the source added, pointing to concerns about Liberty's finances given its recent deal-making.

Liberty, which is owned by U.S. tycoon John Malone, bought Virgin Media in Britain for about $15.75 billion in February and a 13 percent stake in Ziggo in the Netherlands for 633 million euros ($844.9 million) in March.

For its part, Vodafone can finance the offer since its leverage is lower than telecom peers. But ratings agency Fitch said that the group would be downgraded by one notch if it acquired Kabel Deutschland without taking other measures to reduce debt.

Analysts also said on Wednesday that Liberty would be able to reap fewer synergies from buying Kabel Deutschland than Vodafone, possibly hurting the price the U.S. group could offer.

Will Draper, an analyst at Espirito Santo Investment Bank, said for Liberty Global costs could be saved in areas such as billing, customer care, brand, content and R&D that would be worth as much as 8-10 euros per Kabel Deutschland share.

For Vodafone, synergies would be 16 euros per Kabel Deutschland share, stemming from moving its broadband customers in the Kabel Deutschland area onto the cable infrastructure.

"Other benefits which are harder to quantify are the potential for cross selling services and the defence of its mobile business against the possibility of a strengthened MVNO offer from Kabel Deutschland," he said.

Vodafone, Liberty Global and Kabel Deutschland declined to comment on Wednesday.

(Reporting by Harro Ten Wolde and Alexander Huebner in Frankfurt, additional repoeting by Philipp Halstrick; Editing by Leila Abboud and David Stamp)

By Paul Sandle and Sophie Sassard