Campaigning has begun in Oldham East and Saddleworth after it was confirmed a by-election will take place in the constituency on 13 January.
Labour leader Ed Miliband and senior Lib Dems are set to visit on Thursday as parties step up their efforts.
The writ for the poll was formally moved in Parliament.
The by-election was triggered by a court decision annulling Phil Woolas’s victory in May and banning him from politics for three years.
Labour beat the Liberal Democrats by just 103 votes in May’s contest, with the Conservatives in third place.
However, a specially convened election court threw out the result after it found that Mr Woolas had made misleading statements about his Lib Dem opponent.
Mr Woolas challenged the verdict, with an announcement about a by-election date delayed until he recently gave up legal proceedings.
Labour have expressed anger at the shortness of the campaign which will be truncated by the Christmas and New Year holidays. One MP has said the poll is taking place “with unseemly haste”.
However, Labour decided not to oppose the writ – moved by the Liberal Democrats – as doing so would mean the constituency would be left without an MP for more than a year.
Leading politicians from all parties are expected in the constituency over the next few weeks with Prime Minister David Cameron expected to visit during the course of the campaign.
The by-election will be watched carefully as it will be highest profile vote since the coalition government was formed in May.
The Tories and the Liberal Democrats have chosen the same contenders – Kashif Ali and Elwyn Watkins – who represented them earlier this year.
Labour have selected Debbie Abrahams, who came third in the Colne Valley constituency in May.
UKIP chairman Paul Nuttall and BNP leader Nick Griffin also plan to stand.
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