Rhone Poulenc Rorer will have to up Fisons offer, analysts say. (Rhone-Poulenc Rorer Inc.; Fisons PLC)

COLLEGEVILLE - Rhone-Poulenc Rorer Inc. officials want to avoid a bidding war for the British pharmaceutical company Fisons plc, a business it seeks to acquire in a hostile takeover to bolster its worldwide presence in the asthma and allergy medicine market.

Analysts, however, believe the Montgomery County company will have to boost its offer to complete the deal.

On Aug. 18, Rhone-Poulenc Rorer made a hostile bid of $2.6 billion for Fisons, which was rejected as too low by the company the same day.

Fisons, based in Suffolk, England, posted revenues of $1.9 billion and a net income of $82.9 million in 1994.

'FULL AND FAIR PRICE'

"We think what we offered is a full and fair price," said Guy Esnouf, a spokesman for Rhone-Poulenc Rorer. "We hope their board recommends that our offer be accepted."

The day prior to Rhone-Poulenc Rorer's hostile bid of $3.67 per share, …