The liberalisation process has been resisted in some countries and welcomed in others, highlighting different attitudes to competition and the notion of protecting national interests in the energy sector.

The European Commission, which has championed the process, notes that reforms have not been fully implemented in some parts of the EU, meaning a handful of former state energy monopolies still enjoy a crushing grip on their domestic markets.

"It is undeniable that great progress has been made," EU Energy Commissioner Andris Piebalgs said in a recent speech. "However, it is equally undeniable that many of our basic objectives have not yet been achieved."

"Markets remain stubbornly national in scope, cross-border trade is difficult and limited, and far too many customers have little or no real competitive choice of supplier," he added.

In theory, companies have been able to choose a supplier for their gas and power since 2004. Sunday marks the first time the household market has been opened to competition throughout the EU.

Ten of the EU's 27 member states have led the liberalisation vanguard ahead of the deadlines, with Britain setting the trend by opening its markets to competition as far back as 1990.

But the long journey to full market liberalisation, which has taken 11 years to complete, has been marked by frequent battles between the European Commission and more reluctant member states.

The Commission, the EU's executive arm, still believes former state monopolies have too much market power and there is evidence that consumers in some countries are paying too much for their gas and electricity.

To counter this and increase the benefits of the liberalisation process, which is intended to lower energy prices for consumers, the Commission is likely to continue to press for more reforms.