UK Independence Party (UKIP) is following in third place with 11 percent, ahead of the Liberal Democrats, the Scottish National Party (SNP) and the Greens.
ICM interviewed 2,023 individuals across the United Kingdom aged 18 and older by telephone on May 3-6.
The issue of immigration, EU membership and the state of the National Health Service (NHS) are among the most acute issues that have been discussed in the run-up to the general election in Britain.
Polling stations in the country opened at 7 a.m. local time (06:00 GMT) and will close at 10 p.m.
In the absence of a clear leader, the three possible outcomes of the election are a formal coalition, a so-called confidence and supply agreement between the parties, or cooperation on a vote-by-vote basis.
So far, all the parties have spoken against entering into a formal coalition.
