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.
Chancellor George Osborne congratulated Labour leader Jeremy Corbyn on his party's by-election win in Oldham in person after the pair bumped into each other on the train.
Mr Osborne, who was travelling to his constituency of Tatton in Cheshire, was on his way to the buffet car on the train out of London when he spotted Mr Corbyn, who was on his way to Oldham after his party's victory.
The Chancellor congratulated Mr Corbyn on a "very good" result and the two chatted for a few minutes.
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.