Roche will likely up its bid for the 44 percent of Genentech it does not own, analysts said on Tuesday, after the U.S. biotech group urged shareholders to reject the offer.

A special committee of Genentech Inc's board recommended shareholders reject Roche Holding AG's $42 billion bid, saying that at $86.50 per share it substantially undervalued the U.S. company.

Now the ball is back with the Basel company, Wegelin analyst Marco Schwender said. It is clearly possible that investors will now hope for a better offer from Roche.

The Swiss group plans to sell a four-part bond denominated in euros and sterling, Thomson Reuters publication IFR reported on Tuesday. Market talk is that the issue will raise at least 4 billion euros ($5.1 billion).

Roche's surprise hostile offer for Genentech was pitched below the Swiss drugmaker's original rejected bid of $44 billion, or $89 per share. The lower price reflected tougher financing conditions and a drop in the U.S. group's shares.

Roche has said that Genentech's banker, Goldman Sachs, had proposed $112 a share as an acceptable price, giving a glimpse of just how far apart the two sides are.

The difference of opinion highlights the gap between Big Pharma's views -- based on a relatively straightforward financial valuation argument -- and biotech, which uses soft elements like strong science and products in early stages of development, JP Morgan analysts said.

How that gap can be bridged remains difficult to see for us -- hence time to resolution may take longer than expected, they said in a note.

Toward $100 PER SHARE?

We are not surprised to see Genentech refuse Roche's lower offer, said Andrew Weiss, analyst at Swiss bank Vontobel.

We continue to believe that the Genentech asking price of $112 is too high, and that eventually Roche will need to improve its offer to $95-100.

Genentech shares traded up 0.65 percent at $85.19 at 9:37 a.m. EST. Roche stock was down 2.1 percent at 140.20 Swiss francs, versus a 1.5 percent drop in the European healthcare sector.

Prior to Tuesday's bond news, Roche had sold a record $16 billion in debt to help finance the Genentech bid. It has said it plans to use a combination of its own funds, bonds, commercial paper and bank financing for the deal.

It is clear that with Roche raising capital there is no turning back and as such a sweetened offer should be expected, said Karl-Heinz Koch at Swiss brokerage Helvea earlier on Tuesday.

Roche, which won a recent takeover battle for diagnostics company Ventana by sweetening an offer by some 19 percent, has noted the response of the Genentech committee, spokesman Daniel Piller said.

In the Ventana case, Roche sat on its initial hostile offer of $75 a share for some seven months before raising its price to $89.50 and buying the U.S. group for $3.4 billion.

We have laid out our case comprehensively in the tender offer. It is now up to the shareholders to decide on our offer, Piller said.

(additional reporting by Jane Merriman and Natalie Harrison; editing by Sharon Lindores, John Stonestreet)