By Manuela Saragosa
BBC Europe business reporter in Brussels
The European Commission has written to the mobile phone operators Vodafone and T-Mobile to challenge "the high rates" they charge for international roaming.
Vodafone insists the market is competitive
In letters sent to the two companies, the Commission alleged the firms were abusing their dominant market position in the German mobile phone market.
It is the second time Vodafone has come under the Commission's scrutiny.
The UK operator is already appealing against allegations that its UK roaming rates are "unfair and excessive".
Vodafone's response to the Commission's letter was defiant.
"We believe the roaming market is competitive and we expect to resist the charges," said a Vodafone spokesman.
"However we will need time to examine the statement of objections in detail before we formally respond."
Statements of objections
The Commission's investigation into Vodafone and Deutsche Telekom's T-Mobile centres on the tariffs the two companies charge foreign mobile operators to access their networks when subscribers of those foreign operators use their mobile phones in Germany.
Cross-border tourists using mobiles are charged more
The Commission believes these wholesale prices are too high and that the excess is passed on to consumers.
"The Commission aims to ensure that European consumers are not overcharged when they use their mobile phones on their travels around the European Union," the Commission said in a statement.
Vodafone and O2, Britain's other big mobile phone operator, were sent similar statements of objections by the Commission in July last year.
Vodafone sent the Commission a response to those allegations in December last year and is now waiting for a reply.
The Vodafone spokesman said a similar process would be set in motion with these latest statement of objections about its operations in Germany.
The companies will have three months to respond to the Commission's allegations and the process "may go on for some time yet", the spokesman said.
The Commission could charge the companies up to 10% of their annual turnover, though in practice that sort of figure is rarely demanded.
The Commission's latest move comes just a few months after national telecoms regulators across Europe launched a joint investigation which could lead to people being charged less for using their mobile phone when travelling abroad.
The investigation involves regulators assessing whether there is effective competition in the roaming market.