The UK Independence Party will file a formal complaint over alleged "abuses" in Thursday's parliamentary by-election in Oldham West and Royton, leader Nigel Farage has announced.

Ukip failed to make widely-predicted advances in the poll, which saw Jeremy Corbyn pass his first electoral test with flying colours as Labour's Jim McMahon held the seat with a majority of more than 10,000.

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.

The Ukip leader 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".

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."

Mr McMahon polled 17,209 votes, with Ukip's John Bickley trailing in second on 6,487, a majority of 10,722.

Labour's share of the vote increased by more than seven points to 62.1% in the by-election, triggered by the death of former minister Michael Meacher, and there was a 2.27% swing from Ukip to Labour.

Turnout was higher than expected at just over 40%, and Labour's success appears to have been partly secured by an effective postal vote operation.

Mr Corbyn hailed the result as a "vote of confidence in our party", while Mr McMahon dismissed Mr Farage's accusations, saying: "There is nothing wrong with people making a democratic decision not to support Ukip."

Westminster's newest MP - the leader of Oldham Council - said he was "delighted" by the result and vowed to "do my best to live up to those high standards" set by Mr Meacher.

But Mr Farage claimed to have "evidence from an impeccable source that today's postal voting was bent".

Mr Farage said: "As a veteran of over 30 by-elections I have never seen such a perverse result. Serious questions need to be asked."

Numbers of postal votes rose by 15% over the course of the day on Thursday, with individuals handing in bundles of voting slips on behalf of other voters in "processes that shouldn't really be happening in a modern democracy", he claimed.

The Ukip leader cited a newspaper report of households where occupants spoke no English and were unable to identify Mr Corbyn, but said they were voting Labour. He claimed Labour had an advantage in Oldham because of the party's control of council housing and close links with local mosques.

"One of the boxes we saw yesterday was 99% in favour of Labour," Mr Farage told Today. "That doesn't seem to be consistent with modern liberal democracy."

He said he was concerned over a possible repeat of electoral malpractices previously seen in Birmingham and the east London borough of Tower Hamlets.

"It is worrying in terms of how elections work and worrying in the sense of quite where we go with some of our communities," he said.

Last October Mr Bickley nearly overturned an 11,000 Labour majority at the Heywood and Middleton by-election, losing by just 600 votes.

He said Oldham West and Royton was a "different type of constituency with different demographics" in a "very strong Labour heartland".

He claimed the postal voting system was "not fit for democracy right now", while Ukip's deputy leader Paul Nuttall said postal votes had "distorted the result" amid claims Labour had focused on the Asian community.

Mr Nuttall said: "We should go back to the old system where you had to give a good reason why you can't get off your backside and go down to a polling booth. That would make it fair again. That would make polling day actually mean something."

Mr McMahon said: "My sole focus has always been on what is best for Oldham, I want to make our town a better place for my sons to grow up in and make it somewhere they can be proud of, my priority will always be Oldham.

"We also need to remember what is currently at stake under this Tory government. While everyone is looking the other way they are quietly pushing through cuts that will change the face of towns like Oldham.

"The sooner we kick the Tories out and get a Labour government back in, the better for all of us. The hard work starts now."

Mr Corbyn said: "By-elections can be difficult for the party holding the seat, and turnouts are often low. But to increase our share of the vote since the general election is a vote of confidence in our party.

"It's a clear demonstration that Labour is the party working people trust.

"Our determination to oppose Tory austerity policies, and our successes in pushing them back on tax credit and police cuts show that Labour is getting results for working people.

"With the Tories going nowhere in Oldham, Ukip has benefited from a protest vote. But this first electoral test in the new parliament has made clear Labour is the real alternative for Britain."

Tory James Daly was third with 2,596, 9.37% of the vote, while the Liberal Democrats lost their deposit after Jane Brophy secured 1,024 votes, a 3.7% share.

Chancellor George Osborne offered Mr Corbyn his congratulations over the Oldham result in person, after they bumped into one another on a train.

In a tweet, Mr Osborne said: "Good to be one of the first to congratulate Jeremy Corbyn in person on his Oldham win - we're on the same train to Manchester this am."

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.

"I hope our MPs look at this result. What's happened since Jeremy became leader and I became deputy leader is we have focused on issues that affect the working people of Britain.

"I think people responded to that at the ballot box. I hope our MPs will see that if you stand up for working people, they respond by supporting you at elections."