Members of the Richmond and Twickenham Green Party have said they are "deeply disappointed" with Zac Goldsmith for "suggesting" he is endorsed by the party in his campaign material.
Mr Goldsmith, who stood down from his position as a Conservative MP following the Government's backing of a third runway at Heathrow, sparking a by-election to be held on Wednesday, had used a comment from co-leader of the Green Party Caroline Lucas in his campaign leaflets.
The quote from Ms Lucas, which the Greens say has been "highly-edited", reads: "I recognise all of Zac's brilliant environmental work [...] he has been very supportive to me."
But Nicola Albon, spokeswoman for the Greens, said this could "mislead voters into thinking the Green Party supports him in this election", and that the comment was taken from a "personal email" between Ms Lucas and Mr Goldsmith's brother Ben.
The subject of the email is said to have been whether Ms Lucas would support Mr Goldsmith, to which Ms Albon said she had declined.
She said: "While Caroline has worked well with Zac on some environmental issues in the past, she was deeply disappointed by his damaging and divisive campaign to be mayor of London.
"It is completely disingenuous to suggest she is supporting him in this election: she has made very clear that she backs the candidate who is not only anti-Heathrow, but anti a hard Brexit too, and who will use their position in parliament to push for the electoral reform that we so urgently need."
Mr Goldsmith denied suggesting a Green endorsement.
He said: "Caroline Lucas made some generous remarks about my environmental work in parliament, and I was pleased to be able to include them in my literature.
"Clearly I never suggested that she has endorsed me, any more than I suggested John McDonnell, whose quotes I have also included, has endorsed me.
"I was pleased to receive the support of the former Green Party candidate for Richmond Park and North Kingston this morning and to see so many other members of the Green Party locally re-think their earlier support for the Lib Dems."
Ms Albon said the Greens are not officially endorsing a particular candidate, saying members are free to vote for their preferred choice.
She added that although Caroline Lucas, the co-leader of the Green Party, endorsed Liberal Democrat Sarah Olney, this is not official party policy, nor is it the stance of the Richmond and Twickenham Greens.
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