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 Party leader Nigel Farage has claimed the Oldham West and Royton by-election result was "bent" and raised questions over postal votes.
The Ukip leader claimed to have "evidence from an impeccable source"
But the newly elected Labour MP Jim McMahon dismissed Mr Farage's accusations of irregularity, saying "Ukip just need to understand that people have the right to vote for the party that best represents their interests.
"I can say with confidence that we represent every community in Oldham and every community came out and voted Labour."
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.