Steam poured from the cooling towers at the Czech nuclear plant at Temelin early this year.
Some of the 3,000 protesters on the German-Czech border in April urging that the plant be closed.
PRAGUE, July 16 - The German government confirmed today that it has called on its neighbor, the Czech Republic, to shut a nuclear power plant near the German and Austrian borders.
The news sent shares in the main Czech utility crashing and raised new questions about the Czech Republic's plans to join the European Union.
"The German federal government strongly urges the government of the Czech Republic to lift its decision to allow the Temelin nuclear plant to operate, and to close the plant," the German government said in a statement released Friday.
A German government spokesman said Berlin would not use the Temelin issue to block the Czechs' entry to the European Union, but Czech officials fear it will embolden politicians in neighboring Austria, who have threatened to block Czech membership in the 15-nation union unless the reactor is shut down.
Shares in CEZ, the company that owns Temelin, fell more than 20 percent to a two-year low before trading was suspended on the Prague Stock Exchange.
Temelin's two 1,000-megawatt units, designed more than 20 years ago by Soviet engineers and recently upgraded with Western safety systems by Westinghouse, now a unit of British Nuclear Fuels P.L.C., began operating last October, but have yet to go fully on line. They are now sidelined by a faulty turbine.
"This plant is not safe enough to be put into operation," said Michael Schroeren, a spokesman for Germany's environment ministry. "We do not believe this plant can be brought to Western standards of safety. Therefore the best thing one can do is to close down the plant."
Antinuclear sentiment is growing in Germany, and last month German utilities agreed to shut their 19 nuclear plants within 20 years.
Mr. Schroeren said closing the plant was not a condition for letting the Czechs join the European Union, but Czech officials insisted their country needed the electricity Temelin could generate and the cash it would bring in from selling excess electricity to Western Europe.
Temelin's two units, which are years behind schedule and way over budget, have faced fierce protests from local environmentalists as well as Austrians who have blocked border crossings to protest its operation. It opened last October only after an exhaustive safety review by Czech and Austrian scientists.
Further delays or the shutdown of Temelin would savage CEZ, one of the largest Czech companies, and tighten pressure on the Czech government, which is counting on the sale of a 64 percent stake in CEZ to help plug holes in the budget and cut the national debt, now estimated at 24 percent of the country's output.
"It would definitely be bad for CEZ and for the government itself," said Jindrich Svatek, an analyst at Raiffeisen Zentralbank in Prague.