* FDA decision on Tresiba expected around Oct. 1

* Analysts see 80 to 90 percent chance of an approval

* Share price could fall up to 20 percent on rejection

By Teis Jensen

COPENHAGEN, Sept 15 (Reuters) - Analysts are confident the U.S. Food and Drug Administration (FDA) will soon approve Novo Nordisk's new long-lasting insulin drug Tresiba -- just as they were before the drug was first rebuffed two years ago.

Tresiba is considered as a future blockbuster for the Danish company, the Nordic region's most valuable, but the U.S. regulator asked in February 2013 for more tests due to worries it might be linked to higher rates of heart attacks or strokes.

FDA's review of the test results is due around Oct. 1 and analysts surveyed by Reuters see a 80 to 90 percent chance the drug will be waved through this time.

Only a small team of carefully selected Novo experts, none from the top management, gathered the latest test results and held talks with the FDA to avoid information leaks that could potentially bias the ongoing tests.

The team decided in March to submit an interim analysis to the FDA, signalling confidence among Novo's own experts that there is now a strong case for approval.

"We surmise that the Novo team would not have made the resubmission decision if the FDA raised any major concerns during the discussions," Jefferies analysts said in a note, attaching an 80 percent probability of approval.

SHARE PRICE TRIGGER

The share price is expected to drop by 15-20 percent in the unlikely event the FDA rejects the drug, analysts from Jyske Bank, Alm. Brand Bank, SEB and Kepler Cheuvreux said.

But the exact reaction would depend on the FDA's wording and any hints the drug could be approved at a later stage based on more tests, analyst Richard Koch from Kepler Cheuvreux said.

An approval would likely send the shares up a more modest 5 percent, analysts said.

"Even though most people expect Tresiba to be approved, we believe this could turn out to become a trigger with significant share price potential as an approval will remove a major, albeit unlikely risk factor," Nordea said in a note.

Novo is the most valuable company listed in the Nordic region with a total market cap of $145 billion, including its unlisted A-shares. When Tresiba was first rebuffed by the FDA in 2013 the shares fell over 13 percent on the day.

Tresiba is already being sold in 30 countries and analysts, on average, expect annual sales of $2.4 billion by 2020, according to Thomson Reuters Cortellis.

The United States represents some 70-75 percent of global potential sales, Alm. Brand Bank analyst Michael Friis Jorgensen said.

As an ultra-long acting product Tresiba is sold at a premium to other insulins and its approval would also unlock Novo's combination drugs Ryzodeg and Xultophy in the United States.

Novo Nordisk said on Tuesday that new data confirmed Tresiba's benefits in controlling blood sugar levels.

($1 = 6.5989 Danish crowns) (Editing by Sabina Zawadzki and Jane Merriman)