Online operators have used licenses from one European Union member state to try to operate in neighboring countries, arguing that such activity was permitted by the EU's single market.

But EU members like France and Germany, where national gambling monopolies provide a lucrative revenue stream, have tried to curb their activities. While stopping short of the approach in the United States, where some foreign Internet gambling executives were arrested and one was charged, they have moved against several online operators.

The decision Tuesday "will open up the European market, the German market certainly," said Martin Arendts, a lawyer in Grünwald, Germany, who works with a number of gambling companies.

"It's a big step further than the Gambelli decision," he added, referring to a 2003 case in which the European Court of Justice said European countries could not block cross-border gambling operators simply to defend state-run providers, but could do so in order to protect society from the perceived ills of gambling.