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Up to a fifth of the Labour Party’s Thousand Club of donors have quit since Jeremy Corbyn became leader, party sources have revealed.

The exodus is increasing the influence of trade unions whose money is paying a bigger share of the party’s running costs.

The Thousand Club was founded two decades ago by well-heeled backers, with a mission to raise £1 million a year from professionals like bankers, lawyers and investors.

Jolyon Maugham, a top London tax barrister who advised Ed Miliband on his non-dom crackdown before the election, told the Evening Standard he left in September and had switched his financial support to causes like Labour Women’s Network.

“I was giving the Labour Party £5,000 a year but I stopped on the day that Corbyn was elected because of the fact that there was not a single woman on stage with him that day,” he said.

“It has been worse than anyone feared. Once I see Labour with an electable leader again I will no doubt want to renew my support.”

Members give at least £100 a month to join the club, and the elite President’s Tier means giving £5,000 a year or more. In return they are given social gatherings and the chance to meet shadow ministers.

Public relations expert Brian Basham, who founded the club with the late banker Jon Norton who married Mo Mowlam, said he was not surprised that members were leaving: “Regretfully, no, but I would not want to criticise Jeremy Corbyn who is a symbol of change.”

Mr Basham, who left after becoming disenchanted with Tony Blair’s reliance of big multi-million pound donors, added: “I would bung £1,000 into the kitty if I was asked because we need a strong and intellectually-sound opposition.”

Baroness [Jan] Royall, the Thousand Club president and former Labour leader in the House of Lords, said: “It is true to say that people have resigned, but other people have joined.

“With the number of new party members we have seen recently, I would have thought there were potentially many more Thousand Club members.

“It is a great way to support the party’s campaigning, such as next year’s mayoral campaigns and the local elections.”

Several high-value donors vowed to quit if Mr Corbyn won, including Hull Ciy owner Assem Allam who said: “I never back a dead horse”. Insurance tycoon Richard Brindle, who gave £100,000 under Ed Miliband, has branded Mr Corbyn “economically illiterate”.