JEREMY Corbyn has suffered a major blow with the departure of a key aide said to be leaving over concerns about the direction of the party under the current leadership.

Labour sources confirmed that Simon Fletcher was standing down as Mr Corbyn's campaigns chief to pursue "other projects and opportunities".

However, a well-placed Labour MP was quoted as saying that he had been disappointed at Mr Corbyn's failure to achieve his original objective of creating a new type of politics.

His departure comes at an awkward time, just days ahead of two crucial by-elections next week where Labour is fighting to hang onto seats at Copeland and Stoke-on-Trent Central in the face of strong challenges from the Conservatives and Ukip.

Mr Fletcher was among the most experienced members of Mr Corbyn's team, having been the director of his successful leadership campaign in 2015.

He had previously served as Ken Livingstone's Chief of Staff when he was Mayor of London before going on to work for Ed Miliband, liaising between the leader's office and the unions.

Mr Corbyn paid tribute to the work Mr Fletcher had done for the party.

"Simon has played a vital role in shaping the new politics and building Labour's campaign to rebuild and transform Britain," he said in a statement.

"I've known Simon for many years, we will continue to work together on the cause of social justice in Britain."

Mr Fletcher said: "I have worked with Jeremy for well over two decades and will continue to support him and his leadership of the Labour Party and look forward to working together for many years to come."

One party source suggested the senior aide’s departure had strengthened the position of Karie Murphy, who runs Mr Corbyn's office, and who is a close friend of Len McCluskey, the Unite General Secretary.

In 2013, Ms Murphy stepped down as Labour’s Westminster candidate in Falkirk after claims were made that Unite had tried to fix the selection in her favour.

Both she and the union were later cleared of any wrongdoing following an investigation. However, the episode led to Ed Miliband, the then leader, undertaking wholesale party reforms, including electing the leader by a "one member, one vote" system and ending the automatic affiliation of union members.

Meantime, ahead of next Thursday’s by-elections, Mr Corbyn appealed to local supporters in Cumbria and Staffordshire to back Labour and "send a message" to the Government rejecting Conservative austerity cuts.

Speaking on Saturday at Labour's local government conference in Warwick, Mr Corbyn will say both polls will give voters a chance to say they have had enough of cutbacks to health and social care.

In his speech, the party leader will attempt to rally supporters with a call to reject the of attempts of the parties of the Right to sow division in communities.

"Ukip's politics of hate will not save children's centres. Hatred won't build homes. It won't create jobs, and it won't fund health and social care. It won't bring our people dignity or bring our communities together," he will say.

"But that's true of the Tories too. When politicians of the right have no solutions, they find a scapegoat. They try to divide and set people against each other.

"Voters have a chance to tell the Government that they've had enough of their damaging cuts to health and social care and their disregard for human life and dignity.

"Our job, as the Labour Party, isn't to find a vulnerable community to blame; it is to unite people together to build a better world," he will add.