"Today is a very important day," said Martin Selmayr, a European Commission spokesman, at a televised news conference in Brussels. "According to the roaming regulation, all roaming customers in the European Union should be given the opportunity by 30th July, 2007, to opt deliberately for the new Eurotariff."

Selmayr said the European Commission, which has faced resistance from telecom carriers over the law, will collect information on compliance and offer it via the Internet.

"We will put together by the end of this week a Web site which will include the names of operators from all E.U. countries, whether they have offered the Eurotariff, which kind of Eurotariff they have offered, and those who haven't done anything," he said.

The Eurotariff is the product of a new European law that aims to cut the cost of using a mobile phone overseas for the EU's 490 million citizens.

It caps the cost of a call while roaming in another E.U. country at 0.49 euros (US$0.66) a minute and the cost of receiving a call at 0.24 euros. Those rates will fall to 0.46 euros and 0.22 euros in the second year and 0.43 euros and 0.19 euros in the third, respectively.

If carriers don't offer the tariffs they can be subject to legal action.

"This is not a recommendation, it's a regulation. So it's directly applicable law." said Selmayr. "If a customer hasn't received the Eurotariff as of today he can [take the operator] to court."

Monday's deadline is for people to be able to actively apply for the reduced tariffs, which must be activated within one month. Consumers who do nothing should be automatically given the new rates in September.