The Green Party has denied its decision not to field a candidate in the Richmond Park by-election was influenced by the alleged offer of a £250,000 donation designed to help the Liberal Democrats.

The Lib Dems won the seat from the Conservatives in last week’s by-election, unseating former pro-Brexit Tory MP Zac Goldsmith, after the Greens decided not to field a candidate and instead to support the Lib Dems in a “progressive alliance”.

The chairman of the local Green Party branch has sensationally claimed party staff told him not to contest the by-election because of a donation offered to the party which was “conditional” on it not standing against the Liberal Democrats.

In a leaked report, he and two other local members wrote: “Party staff…informed a group of [Kingston Green Party] activists, in confidence, that the party staff were keen for us to agree to stand down.

“This was because there would be serious, but confidential, implications for the national party – so serious that they could even affect the jobs of party staff – in the event that we did not do so.

“Later...it was clarified by party staff (ostensibly on the instruction of the Chief Executive) that this related to a donation offered to the party of some £250,000 which was conditional on the party showing its seriousness about the ‘progressive alliance’ initiative.”

The allegation prompted suggestions the Green Party had taken a bribe in return for supporting the Liberal Democrats.

However, a Green Party spokesperson said the party had rejected the donation and that the offer had not been conditional on it standing aside in Richmond Park.

He said: “This is an unofficial report from three members of Kingston Green Party and is not officially recognised by either the national Green Party or the local party in question. Many of the claims are disputed and this report has been referred to the Green Party Regional Council and will be looked into as soon as possible.

“The Green Party has not accepted any major donations connected with the Richmond Park by-election and no donation offer was contingent on the Party not fielding a candidate. Any donation accepted by the Green Party must meet strict ethical criteria and that includes the donation not being used to gain leverage over strategic decisions.”

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The Greens' candidate in the constituency, Andree Frieze, had decided to stand aside independently and before the donation was discussed with anyone in the local party, he added.

Caroline Lucas and Jonathan Bartley, the joint leaders of the Green Party, had supported calls for a “progressive alliance” in the Richmond Park by-election since “before any donation was proposed”, the spokesperson said.

He refused to comment on the size of the suggested donation or the identity of the donor, saying the party “did not talk about the details of individual donations”.

A source told The Independent the allegations had emerged after a member of Green Party staff erroneously discussed the potential donation with local activists.

The shock Liberal Democrat win saw candidate Sarah Olney overturn the Conservatives' 23,000 majority to win the seat by 1,872 votes.

Mr Goldsmith had triggered the by-election after quitting the Conservative Party over the Government’s support for a third runway at Heathrow Airport.