By Laura Sheeter
BBC News, Riga
Lithuania and Poland have signed an agreement committing them to linking their power grids, thereby deepening EU-Baltic integration.
The Baltic states are integrating their infrastructure with the EU
The so-called "energy bridge" will be the second to connect the Baltic states to the wider EU electricity network.
The first such link - between Estonia and Finland - was opened on Monday.
Baltic leaders are hailing the new connections as historically significant, as they reduce their countries' dependence on Russia.
The three Baltic states, all former members of the Soviet Union, had no connections to the wider European energy grid until this week.
All three countries rely heavily on gas imported from Russia to supply their energy needs, and they feel vulnerable to their large neighbour, especially since the Ukrainian-Russian gas dispute in January.
The opening of an undersea cable between Estonia and Finland on Monday was hailed by the Estonian President, Toomas Hendrik Ilves, as an energy window to Europe, which meant the country's energy security was no longer closely tied to its past.
He said the cable was just the beginning of integrating the Baltic states more closely into the EU's energy network.
The planned "energy bridge" linking the Lithuanian and Polish electricity networks is not expected to be completed until 2010.
The Lithuanian and Polish prime ministers - Gediminas Kirkilas and Lech Kaczynski, respectively - attended the signing ceremony in the Lithuanian capital Vilnius. A joint enterprise is to be set up to build the link.
The Baltic states and Poland are also working on building a new nuclear power plant in Lithuania. It will replace the Soviet-era Ignalina plant, which is due to be shut down in 2009.
The Baltic states joined the European Union in 2004 and see international projects such as these as leading examples of how member states should co-operate on energy. They are among the most enthusiastic proponents of a pan-EU energy policy, which would speak with one voice, especially when dealing with Russia.