Jeremy Corbyn hailed an “incredible” victory after comfortably winning the first electoral test of his Labour leadership.
His candidate Jim McMahon secured a majority of more than 10,700 votes over Ukip and increased the party’s share of the vote in the Oldham West and Royton by-election.
The scale of the victory defied Ukip candidate John Bickley’s promise of a by-election shock and his earlier claims that Mr Corbyn was “toxic” on the doorstep.
Despite predictions that Labour’s majority could plummet below 2,000, Oldham Council leader Mr McMahon took a 62.1% share of the vote – up more than seven points on the late Michael Meacher’s win in the May general election.
Mr Corbyn shared a celebratory hug with Mr McMahon on the steps of Chadderton Town Hall in a whistle-stop visit to the constituency on Friday and told supporters: “It shows just how strong, deep-rooted and how broad our party, the Labour Party, is for the whole of Britain.”
The Labour leader, who blanked questions from the media and stayed in Oldham for less than 10 minutes before being driven away in a car, added: “It shows the way we've driven the Tories back on tax credits, on police cuts, on their whole austerity agenda and narrative.”
But the celebrations were mired by claims from Ukip leader Nigel Farage that he had “evidence from an impeccable source” that the postal voting in the by-election “was bent”.
Mr Farage said he would file a formal complaint over alleged “abuses”. He said he was not questioning Labour’s victory, but denounced the election process after claims from supporters that people had arrived at polling stations carrying bundles of postal votes.
The Ukip leader said the result raised questions about the conduct of elections in areas with large ethnic minority communities.
Mr Farage told BBC Radio 4’s Today programme: “There are some really quite big ethnic changes now in the way people are voting. They can’t speak English, they have never heard of Ukip or the Conservative Party, they haven't even heard of Jeremy Corbyn.
“I'm commenting on the state of modern Britain, post mass immigration. It means effectively that in some of these seats where people don’t speak English and they sign up to postal votes, effectively the electoral process is now dead.”
There were 7,406 postal votes returned in the by-election – 10.7pc of the electorate. Mr McMahon’s majority over Ukip was 10,722.
Oldham Council’s acting returning officer Carolyn Wilkins confirmed on Friday that no formal complaints had been received about alleged postal voting irregularities.
She said the council has a “robust system” for checking postal votes including comparing signatures and dates of birth.
Labour deputy leader Tom Watson said Mr Farage's comments appeared to be a case of “sour grapes”.
“If he has got evidence of that, he should have told the police immediately,” Mr Watson told Today. I have spoken to our organisers and they have got no knowledge of that.”
Mr Watson said: “If this was a referendum on Jeremy Corbyn, then he has won. It was a decisive victory with our share of the vote going up.”
He added: “I hope our MPs will see that if you stand up for working people, they respond by supporting you at elections.”
Abdul Jabbar, a Labour councillor in Oldham, who was among supporters celebrating with Mr McMahon, told The Independent: “I think it’s utter nonsense. If Ukip had any complaint, why didn’t they go to the police or the returning officer. I was at many polling stations yesterday and saw many Ukip representatives and no-one said anything about any malpractice.
“It’s funny that when Nigel Farage lost this election by a very big margin, when they were claiming they were going to win, he comes back with these silly allegations.
“The big issue is they put in an outsider to this place. The reason why Jim won is that he’s a local person and he understands the issues.”
Mr McMahon told supporters he was “overwhelmed” by the result. He said: “I’m very clear. I’m sick to death of what the Conservatives are doing to towns like Oldham.”
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