One of Labour leader Jeremy Corbyn's closest aides is quitting amid concern over the direction the party has been taking, it has been reported.
Labour sources confirmed that Simon Fletcher was standing down as Mr Corbyn's campaigns chief to pursue "other projects and opportunities".
However, London's Evening Standard quoted a "well-placed Labour MP" as saying he had been disappointed at Mr Corbyn's failure to achieve his original objective of creating a new type of politics.
His departure will be seen as a further blow for the embattled leader 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 he 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."