LABOUR’S by-election victory in Oldham shows the party can win back SNP switchers in Scotland, veteran leftwinger Ken Livingstone has claimed, as deputy leader Tom Watson sought to calm MPs’ fears of abuse and intimidation, branding the pro-Corbyn faction Momentum a “rabble”.

Mr Livingstone, the former Mayor of London, said: “What’s so significant is we lost millions of votes to Ukip and the SNP; people who had always voted Labour. What this shows is Jeremy can win those people back. If we had a swing to Labour like this in May, we’d have a Labour government now,” declared Mr Livingstone.

The 70-year-old ex-MP added: “What this will show is all those fears, that a lot of Labour MPs genuinely had, that in Jeremy we had a leader who could not win, that’s now gone…The fact we have done so well in this one is a very strong indicator if you can do that here, we can do that everywhere else.”

On a fleeting visit to the Oldham West and Royton constituency, Mr Corbyn hailed the better-than-expected result as “incredible” and a “vote of confidence” in Labour.

"It shows just how strong, how deep-rooted and how broad our party, the Labour Party, is for the whole of Britain," he said.

Labour’s candidate Jim McMahon, 35, the local council leader, gained a very healthy majority of 10,722 in the Greater Manchester seat on a turnout of 40 per cent.

He polled 17,209 votes with Ukip's John Bickley trailing in second on 6487. Labour's share of the vote increased by more than seven points to 62.1 per cent on a 2.27 per cent swing from Ukip.

The constituency poll was sparked by the death in October of Michael Meacher. At the General Election, the 75-year-old former Environment Minister won a 14,738 on a turnout of 60 per cent.

In the result aftermath, Ukip leader Nigel Farage said he would be lodging a formal complaint over alleged abuses of the postal voting system, saying the result had raised questions about the conduct of elections in constituencies with large ethnic minority communities.

"They can't speak English, they have never heard of Ukip or the Conservative Party, they haven't even heard of Jeremy Corbyn," he claimed.

"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," added Mr Farage.

However, Labour dismissed his allegations as "sour grapes".

Its victory came as some respite for the leadership at the end of a tumultuous week in which deep divisions within the party were blown wide open in the Commons debate on airstrikes on Syria.

Following Wednesday night's vote, there were complaints by MPs of orchestrated email and social media campaigns to pressure them into opposing military action, even though they had been given a free vote.

Tristram Hunt, the former shadow cabinet member, complained of the "reprehensible" intimidation of female MPs while backbencher Neil Coyle - who referred an alleged death threat to the police - urged Mr Corbyn to distance himself from Momentum.

The grouping, however, strongly denied plotting to deselect MPs, who had backed airstrikes while Mr Watson said the group's influence had been overstated.

"They look like a bit of a rabble to me but I don't think they are a problem for the Labour Party. I don't think they are effective. They are a bit of an irrelevance in the debate," he said.

The deputy leader also had some sharp words for Mr Corbyn's critics within the party, saying some of their comments had been "deeply unhelpful" and called on them to "swing together" in the wake of the by-election victory.

"If this was a referendum on Jeremy Corbyn, then he has won,” declared Mr Watson.

"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. People responded to that at the ballot box," he added.