Zack Polanski has said he was wrong to describe himself as a British Red Cross spokesperson, and that intensified media scrutiny of the Green party reflected fears of its rising popularity and support for wealth taxes.

Polanski described himself as a British Red Cross spokesperson while campaigning for the party leadership, the Times revealed. The claim was also mentioned on his personal website in 2020 when he said he was “really proud of the work we do”.

The British Red Cross said Polanski had not been a spokesperson for the charity, and that it had raised the issue with the Greens.

Polanski also hit back at what he characterised as politically motivated attacks on his party, accusing rightwing media owners with wealthy interests of fearing the party’s growing support.

He told BBC Radio 4’s Today programme: “I hosted various fundraisers for the British Red Cross, and indeed I would go on stage and speak for them about the amazing work they do tackling humanitarian crises, on the climate crisis, and indeed, for refugees all around the world.

“I used the wrong word, and I accept that, but I would essentially take words on stage with me and speak. It’s important, though, and I accept this, that they don’t support any political party, and I’ve made sure that’s been taken down.”

He also accused the Times of unfairly targeting him. “Well, I think it’s totally fine to ask me questions about my past,” he said. “I would also say, in the same breath though, the Times published a pretty antisemitic cartoon of me last week. I asked them to apologise, and it feels some of these stories feel like scraping the barrel to kind of go back 10, 15 years.”

Polanski claimed the Greens’ electoral rise had worried parts of the political and media establishment. “People who own rightwing media, multimillionaires and billionaires … are worried about the prospect that they might have to pay a little bit more tax,” he said. “We had 50,000 members. We’ve now got 225,000 members. So we are rising.”

Polanski was interviewed on the final day of campaigning for local elections in England and parliamentary elections in Scotland and Wales on Thursday, with Labour braced for a very difficult night when the results come in.

He also defended the Greens against antisemitic comments made by several of its candidates. Two running for Lambeth council in south London, Sabine Mairey and Saiqa Ali, were arrested on suspicion of stirring up racial hatred online.

“I am responsible,” Polanski said. “Those messages are all unacceptable and it’s important to condemn that.” He also said the party would introduce a standardised vetting process and compulsory antisemitism training for all of its candidates.

He said that would “make it clear that antisemitism is completely unwelcome in the Green party, as it is in society”. Describing the Greens as an anti-racist party, he said: “It is also important to say one case of antisemitism is one too many. This is a handful of cases and actually we have over 4,500 candidates, the vast, vast majority of which are doing amazing work in their communities right now.”

Asked how he would avoid becoming “the new Jeremy Corbyn of British politics”, Polanski said they were “very different people” but that Corbyn had put forward lots of policies that were “really positive” for governing, including wealth taxes and public ownership.

Polanski said he was not “ready right now” to become prime minister, given he had only been party leader for eight months, but that he would “certainly be putting in the work” over the next few years. “There’s lots of skills and lots of knowledge to get, and I think that’s fine,” he said. “I’m a human being. I’m not perfect.”