LONDON — A key ally of Labour leader Jeremy Corbyn has warned that the party is in danger of being "routed" by UKIP in their Northern heartlands.

Shadow Business secretary Clive Lewis said that Paul Nuttall was close to winning the upcoming Stoke-on-Trent Central by-election, with the party "hanging on by the fingernails" in large parts of the country.

"There are swathes of this country, like in Stoke, where we are hanging on by the fingernails to keep UKIP at bay," he told a meeting in his constituency on Friday.

"And if UKIP make a breakthrough in Stoke and parts of the North there will be a rout."

The Stoke-on-Trent Central by-election is currently on a knife edge according to bookmakers, with UKIP leader Paul Nuttall joint favourite with Labour to claim the seat.

Lewis said that defeat to Nuttall would cause many other previously safe Labour seats to fall.

"Once they have one voice they will have a base and it will be a domino effect," he said.

However, despite warning about the rise of UKIP, Lewis told the meeting that he still may vote against Article 50.

Lewis, who represents Norwich South, which overwhelmingly voted for Remain, said he could be forced to resign from the shadow cabinet if Labour fails to win its amendments to the Brexit bill this week.

"If at the end of that process the bill before us is still an overwhelmingly Tory, hard, cliff-edge Trumpian Brexit then I am prepared to break the whip and I am prepared to walk from the shadow cabinet," he said.

Labour have yet to decide whether MPs will be whipped to vote in favour of Article 50 being triggered, or whether frontbenchers who defy the whip, would have to resign.

However, shadow foreign secretary Emily Thornberry insisted on Sunday that Labour would not do anything to block or delay Article 50 being triggered.

"We have said that we will not frustrate Brexit," she told the Sunday Politics.

"We have got our instructions from the British people. We are democrats and the public have voted to leave the European Union."

Asked on Sunday what action would be taken against Brexit rebels on his frontbench, Corbyn told Radio 4’s The World This Weekend: "I’m a very lenient person."

Shadow chancellor John McDonnell also suggested frontbenchers could be spared.

"We’ll see whether they keep their jobs, whether they stand down, or whether they get moved," he told Pienaar's Politics.

"We're keeping that open."

McDonnell also suggested that Labour would overtake the Conservatives in the polls once Labour's divisions on Brexit had passed. In recent weeks polls have put Labour up to 17 points behind the Tories.

"Once we get past Brexit we’ll unite the Labour party we’ll be back on our agenda, and you’ll see I think that we'll have a significant impact on the political debate in this country which means we’ll go back in front of the Tories in the polls over the next year," he said.