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.
Ukip leader Nigel Farage has said his party will make a formal complaint after claiming the Oldham West and Royton by-election was "bent."
Mr Farage said he was not questioning Labour's victory, but denounced the electoral process as "bent" after claims that people had arrived at polling booths carrying bundles of postal votes.
He said the result raised questions about the conduct of elections in areas with large ethnic minority communities and claimed that in constituencies with large numbers of minority voters who do not speak English "effectively the electoral process is now dead".
"As a veteran of over 30 by-elections I have never seen such a perverse result. Serious questions need to be asked," Mr Farage said.
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.