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One of Jeremy Corbyn’s closest aides is quitting over Labour’s direction under its embattled leader, the Evening Standard has learned.

Simon Fletcher is standing down as Mr Corbyn’s campaigns chief, which will plunge the party into further turmoil.

Labour insiders said the senior party strategist had grown increasingly concerned because his early hopes of a fresh direction for the party had disintegrated.

His departure is a heavy blow for Mr Corbyn, following frontbench resignations over the Brexit vote and plummeting poll ratings.

The resignation comes ahead of another challenging week for the Labour leader, as the Brexit bill goes to the House of Lords and his party fights to cling on to seats in the Stoke-on-Trent Central and Copeland by-elections.

Official Labour sources said Mr Fletcher, who was previously Mr Corbyn’s chief-of-staff, wanted to focus on other projects.

However, a well-placed Labour MP revealed he had been disappointed that Mr Corbyn had fallen short of his early objective of a new type of politics. Mr Fletcher declined to comment.

The 48-year-old Left-winger was unusual among Mr Corbyn’s inner circle in that he commanded the respect of MPs across the Labour spectrum, rather than just the leader’s own faction. Seen as a clever and capable pragmatist, he was credited by many MPs with showing a grasp of political reality that some in the leader’s office appeared to lack.

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

Mr Fletcher, who will leave Mr Corbyn’s office today, earned his spurs as Ken Livingstone’s chief of staff at City Hall. When the former mayor was out of town, Mr Fletcher was left in charge of the capital.

When Boris Johnson ousted Mr Livingstone from office in 2008, Mr Fletcher 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 puts Labour at 24 per cent, trailing the Tories by 16 percentage points. If a general election were called, the Tories would win with a huge majority.

Mr Corbyn’s personal approval ratings are low, but there has also been widespread criticism of the leader’s office, with key lieutenants accused of causing many of Labour’s current woes.

Many Labour MPs feel that Mr Corbyn’s political operation is often incoherent, while party insiders complain about the blinkered approach of some of his senior aides.

The party has been casting around for a successor to Mr Corbyn in recent months, with shadow Treasury spokeswoman Rebecca Long-Bailey and former shadow business secretary Clive Lewis both tipped as favourites for the job.