Buwal, the federal environment office, said it had turned down a request by Pluess-Staufer AG to test T25 maize as well as a petition by the federal research institute RAC to test transgenic potatoes. It cited health and environmental concerns.

"In Buwal's opinion, there is insufficient evidence that there is no need for concern for humans and the environment," it said in a statement.The decision can be appealed within 30 days.

Hans Hosbach, head of Buwal's biotechnology and flux of substances section, said the decision made non-EU Switzerland a unique island within Europe, where most states including neighbouring Germany and Italy permit growing genetically modified crops.

"Switzerlandhas now said: `No. We don't want this,'" Hosbach told Reuters.

Buwal said it turned down the maize request due to concerns over inadvertent pollination. It rejected the other tests out of concern the genetically modified potatoes contained genes which could make some bacteria resistent to certain antibiotics.

"We are setting extremely high standards from the start. The two requests did not meet those standards," Hosbach said.

The Buwal statement said the ruling also sought to protect the image of Swiss products.

"Swiss agriculture lives on our products' reputation for being pure and close to nature. Such gene technology experiments affect this image. This can have a far-reaching impact on our agricultural sector," the statement said.

While the ruling only covers the two specific cases for maize and potatoes, it does set a certain precedent, Buwal spokesman Rolf Wespe said.

"To some extent the case sends a signal, but the matter has to be decided on a case-by-case basis," he said.

T25 is made bya joint venture of Germany's Hoechst AG and Schering. Officials at the companies were unavailable for comment.

Environmental group Greenpeace said it was "thrilled" by the decision, saying the tests would have been dangerous and unnecessary.