* Federal court backs last year's decision by Hesse court
* RWE could claim 187 million euros -industry analysts
* Shares rise 4.3 percent (Adds RWE spokeswoman, environment ministry, context)
By Christoph Steitz and Tom Käckenhoff
DUESSELDORF/FRANKFURT, Jan 14 (Reuters) - Germany's No.2 utility RWE is preparing to sue for millions of euros of damages after a federal court confirmed that a state's decision to shut down the company's Biblis nuclear plant for three months in 2011 was illegal.
A spokeswoman for RWE said it planned to take action against the German state of Hesse, which ordered the closure of Biblis, Germany's oldest nuclear plant, as a precaution following the disaster at Japan's Fukushima plant nearly three years ago.
"Preparations for a lawsuit are being made," the spokeswoman said on Tuesday.
RWE is the only utility to have taken legal steps over three-month shutdowns, which the states of Hesse, Lower Saxony, Bavaria and Baden-Wuerttemberg imposed on Germany's seven oldest nuclear plants at the time.
RWE had filed a complaint in April 2011 against Hesse. Last February, the Hesse Administrative Court said the order had been illegal, a decision that was backed by Germany's Leipzig-based Federal Administrative Court on Tuesday.
Hesse's Environment Minister Lucia Puttrich said the court's ruling could not be seen as a preliminary decision on whether RWE could assert damage claims against the state of Hesse. This would have to be determined in a separate suit, she said in an emailed statement.
Shares in RWE, Germany's second largest utility by market value after E.ON, rose after the news and were up 4.4 percent, topping the German benchmark DAX index.
The Fukushima disaster later also triggered Germany's decision to exit nuclear power by 2022, forcing Germany's top utilities - E.ON, RWE, EnBW and Vattenfall - to book billions of euros of writedowns and cut thousands of jobs.
E.ON, RWE and Vattenfall have filed constitutional complaints with Germany's highest court against this decision, claiming billions of euros in compensation. A decision could come as soon as this year, the court has said.
"The decision of the Federal Administrative Court has to be respected. But the decision does not change the fact that pulling out of nuclear power is irreversible," a spokesman for Germany's Environment Ministry said.
The spokeswoman for RWE declined to comment on the potential size of the claims over Biblis but industry analysts have estimated that RWE suffered about 187 million euros ($255 million) in damages as a consequence of the forced shut-down.
Biblis has remained idled since 2011 largely as a result of Germany's wider decision to phase out nuclear power.
RWE estimates that dismantling its two reactors at Biblis will cost 1.5 billion euros, excluding storage costs for the nuclear waste. ($1 = 0.7324 euros) (Editing by Anthony Barker)