By John Moylan BBC Europe business reporter in Brussels

The initial finding by the European Commission followed a lengthy investigation into the amount customers from abroad were charged when roaming on Vodafone's or MM02's network in the UK.

When you travel to a foreign country, your mobile phone will automatically find a local network, allowing you to make calls. The system is known as roaming.

Europe's competition authority has long been concerned about the level of roaming charges. In 2001, it conducted raids of mobile phone operators in the UK and Germany, two of Europe's most important and advanced mobile markets.

Now it's concluded that in the period between 1997 and 2003 Vodafone and MM02, formerly part of BT, enjoyed a dominant position in the UK.

German probe

What's more, the prices faced by foreign companies to let their consumers roam in the UK were much higher than those faced by UK firms such as Virgin which don't own a network but buy air time off others.

The two companies have now been sent formal statements of objections and have two months to respond. If the finding is upheld they could face fines of tens of millions of pounds.

Investigation into the German market is still ongoing and could lead to similar conclusions.

The mobile operators say roaming charges have already come down.