Jeremy Corbyn’s leadership has been plunged into fresh turmoil by the sudden resignation of one of his closest aides.

Campaigns chief Simon Fletcher stood down after what appeared to be clashes with Mr Corbyn’s inner team over Labour’s direction.

The departure follows the resignations of four shadow Cabinet members over Mr Corbyn’s support for the Article 50 bill amid plummeting poll ratings.

And it comes just six days before two crucial by-elections – in Stoke-on-Trent Central and Copeland – which are widely seen as must-wins for Labour.

Officially, Mr Fletcher, who was previously Mr Corbyn’s chief of staff, was said to be keen to focus on other projects.

However, one well-placed Labour source said Mr Fletcher was frustrated by an inability to force through necessary changes against internal opposition.

The 48-year-old is a left-winger but, unusually among Mr Corbyn’s inner circle, enjoyed the respect of Labour MPs of all viewpoints, rather than just the leader’s own faction.

The source said: “Simon leaving is a big blow to the party and I think Jeremy will be very sorry to see him go.

“He believed Jeremy could be the ‘change’ leader that he wants to be, but he wasn’t able to do the things that he wanted in order to make that happen.

“Most of the people around Corbyn have never been involved in running an organisation. Simon was the one person who had.”

Mr Corbyn’s inner circle is dominated by communications chief Seamus Milne, policy adviser Andrew Fisher, political secretary Katy Clark and Karie Murphy, who runs his office.

Mr Fletcher – who declined to comment – was Ken Livingstone’s powerful chief of staff when he was London’s mayor.

He left frontline politics until 2013, when Ed Miliband hired him to liaise between the Labour leader’s office and the trade unions.

The latest nationwide poll put Labour at a dismal 24 per cent, trailing the Tories by 16 percentage points and at risk of a general election disaster – should one be called.