Jeremy Corbyn hailed the Oldham by-election win as a "vote of confidence in our party" after Jim McMahon swept to victory for Labour.
McMahon secured a majority of more than 10,000 and an increased share of the vote in the by-election which was sparked by the death of long-standing MP Michael Meacher.
But Ukip leader Nigel Farage complained of a "bent" contest and said the party would file a formal complaint.
Deputy leader of the Labour party Tom Watson has dismissed Nigel Farage's complaints over the by-election as "sour grapes."
The Ukip leader claims the result was "bent" over concerns over postal votes.
But Mr Watson said Farage had said the by-election would be "a referendum on Corbyn" and the Labour leader had "passed with flying colours."
Jeremy Corbyn hailed the Oldham by-election win as a "vote of confidence in our party" as Jim McMahon held the seat for Labour.
A by-election in the historically safe Labour seat of Oldham would, ordinarily, be a foregone conclusion. These are far from ordinary times.
The Oldham West and Royton byelection will be a fascinating crash test - as Corbyn's Labour comes up against Ukip's northern force.