An analysis by Britain's main television networks suggested David Cameron's Conservative party will win 307 House of Commons seats, short of the 326 seats needed for a majority.
Polls gave Prime Minister Gordon Brown's Labour Party 255 seats, and Nick Clegg's Liberal Democrats 59 seats - far less than had been expected. Small parties got 29 other seats.
The result would bear out predictions that this election would not give any party a majority, resulting in a destabilizing period of political wrangling and uncertainty.
Two scenarios could arise - Brown could resign if he feels the results have signaled he has lost his mandate to rule, or he could try to stay on as leader and seek a deal in which smaller parties would support him.
Even combined, Labour and the Liberal Democrats would not have the 326 seats needed to form a majority in a coalition - which had been a widely discussed possibility.
The results may yet change. Projecting elections based on exit polls is inherently risky - particularly in an exceptionally close election like this one. Polls are based on samples - in this case 18,000 respondents - and always have some margin of error.
Thousands have also already cast postal ballots but those results don't factor into the exit polls. About 12 percent cast postal ballots in 2005.
Britain's census is nine years out of date and the polling districts haven't caught up to population shifts. Many voters also refuse to respond to exit polls.
The projection suggests that the Conservatives will gain 97 seats, Labour lose 94 and the Liberal Democrats lose three.
"I think we're going to see a very interesting night," Conservative Party chairman Eric Pickles said.