Counting began in parliamentary by-elections in Manchester and Cardiff after polls closed on Thursday night but the results of a crunch vote in Corby will not be known until Friday afternoon.
The Northamptonshire seat is a major by-election test for David Cameron, with the Conservatives defending a slim majority of fewer than 2,000.
Ed Miliband's Labour Party has established a solid poll lead since the last election and the Tories' campaign was plunged into controversy on Tuesday night when it emerged their campaign manager Chris Heaton-Harris had encouraged a rival candidate.
The by-election was caused by the resignation of MP Louise Mensch who will move to New York with her family.
Votes in Cardiff South and Manchester Central were not expected to produce an upset, each carrying a strong Labour majority from the last election.
Manchester Central, with a Labour majority of more than 10,000 from 2010, was vacated by Tony Lloyd so that he could stand in today's Police and Crime Commissioner (PCC) elections.
Cardiff Central, where Labour won by just over 4,700 votes two-and-a-half years ago, was prompted by the resignation of another PCC candidate, Alun Michael, who has held the seat for 25 years.