Farm, glue and Experience won in a review that included two other agency teams: Harrison Troughton Wunderman, LBi and Manning Gottlieb OMD; and DFGW, Red Nomad and MindShare.
The three winning agencies will be contracted to increase awareness of voting systems over the next three years. The £5.5 million budget may increase in this time, according to the frequency of elections.
The review comes as the Electoral Commission moves to target voter apathy. Turnout at the 2005 general election was 61.4 per cent, up 2 per cent on the 2001 peak, but 10 per cent down on 1997.
The Electoral Commission's campaign manager, Nina Morris, said: "We're delighted to appoint Farm Communications, glue London and Experience.
"They devised impressive creative ideas for encouraging people to register to vote and identifying effective ways to reach those least likely to be registered."
In August, the Government revived plans to permit votes to be cast online in a bid to reverse the falling turnouts in elections, by making voting more convenient and relevant to younger voters.